As it says above, I would like some feedback on making my encounters more of a challenge. I have 4 level 7 players. They will come across hobgoblins and the variants but a decent majority of hostiles will be goblins, however I want them to see more organized/militarized.

for them fighting the generic goblins with that stat block is pretty easy and I would like to make it tougher and more thrilling. Ways I could this:

double the hps I roll for goblins, give some shields to increase their AC. However I am also considering adding class levels. Fighter and barbarian melee goblins, ranger archer goblins. That being said, they would gain the class actions for those. 
I am looking for suggestions of what level I should have them beat to make it more challenging for the players. I’m think level 3 for the goblins could work out alright. But would like feedback as well. Thank you

Instead of trying to force goblins to be tougher than they are, use a different monster and re-skin it. Use stats for trolls or ogres and just describe them as goblins. I realize there’s a size difference, so that’s maybe not the best example, but you get the idea.

As a bonus, it helps confuse players who might know the monsters and lets you avoid metagaming. “The goblin is regenerating? That’s not supposed to happen!”

I think goblin numbers are going to be important.
Draining players key resources(spells, action serge, etc) before encountering goblin is a way to make it more challenging.

If you're outnumbered and don't have AoE spells like fireball, even goblins can be in trouble.
upgrade 'AC' is not fun for me make monster "glass cannon" adding damage is more fun to me and my player as well

Please note that this is my personal experience, so it may not work for you.

Give the goblins functional class levels. There's a bunch of generic classed NPC stat blocks and they can just be goblins. 

Give them pets.

Ultimately, you're going to have to reckon with the fact that goblins are a low-level threat, and you'll need to move on and mix things up to avoid stretching credulity with all the high-powered goblins they're now fighting. (This isn't specific to goblins. Any playable species runs into the same problem. One of the ongoing plots in my game involves an evil cult, but if it was all they fought, the endless supply of high-level cultists would be a problem.)

Funny you should post this.  I am also planning a goblin encounter for my group of 4 level 7 players.  It is a side quest/one shot that will have them retrieving pets.  Long story on the pets but basically a couple new players wanted them badly.  The party will also find that the goblins have captured the captain and crew of the ship that brought them to the current island they are on, and will need them to sail back home.

Anyway - the party will track them into a cave system.  Along the route there will be a few traps with a dozen goblins in wait.  Inside the caves, there will be parallel tunnels dug by the goblins with arrow slots.  One tunnel will be a dead end, with a cave in that will trap the party, then a burning cart will be pushed toward the party.  There will be many small tunnels that a human/dwarf can’t fit into that the goblins can move quickly in.  One tunnel will have holes in the ceiling for pouring oil onto the party.  They will use almost exclusively short bows and use a bonus action to hide. Since goblins are lightweight, there will be some stone bridges that collapse under a human's weight.  There will be a room with wolves that are used as mounts, and the goblins will make attacks while riding.   Boss will be a bugbear chief.  

I think having them use bonus action hide, shooting from cover and using larger numbers will make it challenging.  

Still heavily in development but will be ready in a couple weeks when we play again. 

Even simple goblins can be a lot in waves. Sheer numbers can really stress a player. Even if they can one-shot a goblin, if there are twenty more on the horizon, was that really a victory? Some times hoards can do more than a single target can accomplish. Im not a fan of just boosting HP. Damage sponges arent fun. Turn this into Dynasty Warriors and give them dozens to mow down and drain those resources. Just give them a slightly higher damage output but keep them just as squishy.

There are a few ways to make goblins more challenging

Home field advantage

Goblins have some differences from the majority of humanoids that they can take advantage of especially when choosing/ building a home base

• Dark vision: They would not need to have lights making their base dark. They can also take advantage of the fact dark vision only upgrades darkness to dim light to hide traps. 
• Small size: They could make passages small enough to disadvantage medium creatures but not small ones. They can create passages too small for medium characters to squeeze through or create areas that give a combat disadvantage due to the size of the space. https://www.dndbeyond.com/sources/basic-rules/combat#SqueezingintoaSmallerSpace
• Stealth: Goblins are good at stealth they would create blind corners and small nooks to hide in for ambush's 

Equipment

As humanoids goblins can use any equipment the players could

• Explosives like bombs and dynamite can provide a new type of ranged attack or could allow for things like self destructing. 
• Nets goblins can team up to use nets to great effect.  They can walk in throw the net and disengage to leave as a bonus action. This would give all their allies advantage on their attack which can be devastating
• Poison can add extra damage of give an attack the ability to debilitate. Having goblin with a blow dart can be an effective way to create a priority target 
• Whips and pole arms give goblin melee attacks reach . You can combine it with the sentinel feat to create goblins that whip players when ever they try to move to keep them in place. Goblins are better at using whips than pole arms but polearms also have the polearm master feat which will allow them to stop enemies from approaching. If you want could also allow this feature on whips 
• Alchemists fire gives some goblins yet another unique attack

Pets
Goblins can have pets and mounts

• Mounted goblin wolf riders is a classic
• Rust monsters live in a similar environment and are terrifying to players. Goblins probably don't need plate male sized for humans so they can feed to them. 

Magic  

• nilbog is a magical enemy that presents a unique challenge jumping from goblin to goblin and preventing damage 
• goblins drinking random poitions and using random magic items not knowing what they do is always fun 

I'm thinking along the same lines as jl8e.

Instead of going with the stat block goblin, give the goblins random character classes. Goblns can be PC characters, which means they can be fighters, rogues, wizards, sorcerors, warlocks, etc...

Roll up some generic 'goblin' class stats and start throwing PC classed goblins at them. Nothing says 'love' like a 5th level goblin warlock that's eldritch blasting the stuffing out of everything in sight...or the raging 7th level goblin barbarian with GWM and 2 attacks...

Basically stated...think outside the box on your encounter creatures and don't always just go with the stat block.

Change the environment. You don't have to change the stats of the goblins. Just change the battlefield. Pits, chasms, bridges, nets, balconies, oiled slopes, illusions, mirrors...