Plz know that all ideas would be performed by a 5th lv Artificer. 

1. Can an Artificer create a +3 shield? if so, how long would it take?

2. Can an Artificer make Adamantine armor? If so, how long would it take?

3. What magic items (other than +1, +2, and +3 armor and shields) can my Artificer create to boost a creatures AC?

As far as I'm aware nothing RAW gives an Artificer the ability to make any of that apart from Infusions, which are limited to +1 shields/armor at that level, and max +2. The Replicate Magic Item infusion lets you make other items but at 5th level that is very limited.

Making Adamantine armor/shield +3/non-armor items that effect AC would be up to your DM, especially at level 5.

There are a lot of infusions, and my phone doesn't  let me reliably swap pages without wiping what I've written. I'm also not allowed to link to a page, so I can't direct you to a list either. I know Enhanced Defence gives you  +1AC for armour or shield, rising to +2 at L10.

Other than infusions, the same rules apply to Artificers as any other class when making Magic Items, I not sure that you can make +X items by RAW. I'd insist that you can't make a +X item that is rarer than you could expect to find in loot at your level - an L1 character isn't going to be able to forge a Legendary Weapon. So at L5, that means Common or Uncommon. It's pushing it for Rare because you've only just qualified for any at all. That means you can make +1 weapons, and if you're really desperate, I might be willing to consider +2, but doubtful. +3 will earn a "Pull the other one!". You'd have to be level 7 or more for me to be comfortable with +2, at the very least +L11 but probably more likely L13 for +3. I'm going by guidance I've received by someone else on the board of when magic items stop being game distorting.

I assume you're talking about crafting magical items when you talk about creating items to boost a creature's AC, and that you're either not interested in using the Artificer's infusions to do so, or are already doing so and want extra protection.

As Horu pointed out, there's tables in the DMG and in Xanathar's Guide which give an estimate of cost and work time for creating magical gear.  That would be what a DM running the game RAW would use.  Under those tables, it's very difficult for a fifth-level character to create Rare or higher items.  Very Rare items, such as a +3 shield, are right out.  The most cost-effective means of boosting AC for a fifth-level magic item crafter are the shield, +1 and the cloak of protection, both of Green/Uncommon rarity.  Assuming you have steady, reliable access to the Secret Ingredient your DM deems necessary for each item, you could produce five Green items in the worktime it takes to produce one Blue, and you'd make those five items for half the GP cost.  You could equip your entire party with +1 shields and Cloaks of Protection for the cost/time of a single +2 shield, for the same +2 AC bonus as well as a +1 bonus to all saving throws.  Unless your fifth-level party is somehow already full up on attuneables (wut), there's no reason to ever craft a +2 shield when you could craft a +1 shield and a Cloak of protection instead.

Adamantine armor can be crafted, but I can't imagine a DM who wouldn't say you need a source of adamantine first.  If you have it?  Adamantine armor counts as a Green/Uncommon item and would take 200gp (plus the cost of the armor you're making) to craft, and expend two weeks of time (plus the time spent making the armor).  Xanathar's Guide states it takes one week per 50gp of base cost to craft a mundane item.  A shirt of adamantine scale mail, thusly, would require 25gp of mundane materials, 200gp worth of adamantine, and three workweeks if the crafting was run as close to RAW as possible.  That would be your most cost and time-effective way of creating adamantine armor, though do note that adamantine armor does not increase your AC.  It renders you immune to critical hits, which can be a huge defensive benefit, but it's not an AC boost.

Assuming you're also making the mundane shield and the mundane cloak you need for the +1 and the Protection cloak from scratch, assembling the whole kit - +1 shield, Adamantine scale mail, and a Cloak of Protection - would require eight to nine workweeks, three Secret Ingredients acquired by defeating threats/harvesting resources from CR 4 to 8 challenges with one of them being a source of adamantine, and roughly 700 gold.  That's a hefty investment for most typical level 5 characters, but it's not out of the question.  And the resulting package of protectives is quite nice indeed for a level 5 to 7ish artificer, or anyone else you care to bestow the fruits of your labor upon.  If you care about stealth you'd have to upgrade the scale mail to a breastplate, which would add seven additional workweeks and 175 additional GP worth of materials, since apparently breastplates are a Lost Secret Art next to scale mail, but you'd lose that stealth disadvantage.  Frankly, unless your campaign involves a lot of skullduggery I wouldn't bother, the scale mail's much easier on the pocketbook and more importantly the time card for someone interested in fabricating multiple items in quick succession.

Awesome thx everyone for the info

Weighing in a little late but here goes. As an artificer you have 3 broad ways of making materials:

1) As a mundane skilled worker - so assuming you have the right skills and tools (probable) to do so you can make mundane armor/weapons/items following the rules in the various books.

2) As an artificer - you have your infusions but they have some very basic limitations:
A) no infusion can be in more than 1 item at a time - so you can’t infuse a bunch of enhanced shields for your party but you can do a single one for your self.
B) you have a limited number of infusions you know and can have active - at L5 that is 4 infusions known and 2 active (at best at L20 the armorer knows 15 infusions and can have 8 active - 2 of which must be on parts of a suit of armor including chest, helmet, gauntlets and boots)
C) you are limited in how much enhancement you can provide - unless you are an armorer all you can do with infusions is a +1. Armorers get a subclass feature at L10 that increases that to +2 but that is their max.

3) as a magic wielder of some sort you can, at least in theory, actually create true magical (enchanted not just infused) item just like a Wizard. However, you are a half caster so you are limited to a max of L5 spells so any item requiring a spell or spell slot above that is probably going to be impossible (a multiclass with a full caster might have high level slots but no spells) this comes into play with spells like magic weapon - L2 slot (base) gives you a +1 weapon (for a while), using a L4 slot makes the weapon +2,. But to get a +3 weapon you would have to have a L6 slot available. So by analogy creating a +3 armor/shield is probably going to take a L6 slot as well and you certainly won’t at L5 (the earliest you could get it would be L14 as a L6 artificer/L8 full caster) (of course at that point why go artificer?)

so - a +3 shield - no way; Admantine armor - maybe- do you have a source for the Admantine? Do you have smithing/smiths tools? If yes then you can following the times and costs in the rules books; other items? Pretty much just those that you could do by infusion as that list pretty much outlines what you could do at best by enchantment also. 

So, a +3 Shield is a Very Rare item, and according to XGtE, crafting a Very Rare item takes 25 workweeks and costs 20,000GP in resources, and you need access to a forge or other crafting supplies throughout. You also need to have some form of Plans for how to do this... it's not something your character can just make using their intuition. That could mean finding instructions on how to create this item, or being instructed by a skill craftsperson, or potentially even adding the time and resources of doing research and experimentation on your own in addition to the time/costs of crafting.

25 workweeks is, almost literally, half of a year of labor.

If you happen to have 20,000+ GP sitting around and you're about to get the better part of a year as downtime, sure, it's something you could do. But keep in mind that the time/cost growth is exponential... so while a Very Rare magic item costs 20,000 GP and half a year of work, a Rare item such as a +2 shield costs a comparitively small $2,000GP and only 10 workweeks of time. Still something you're going to need extensive downtime to craft, but lowering your goal just a little bit makes it massively more achievable.