Hi my friends, 

So I Am Dming for long yerars for mostly other games then DnD so I am not that knowledgeable about Dnd 5E monsters. 

I do have group of  4 lvl 3 players ( bard , paladin , warlock, wizzard, druid) where bard  is rather experienced dnd player , druid is new to dnd but experienced TTRPG overall and rest is mostly inexperienced.

as they are in campaign so far they did massacred some undead and bandits. Some bigger beasts which i thought that would be challenging encounter for them but was not .

So any ideas on an interresting and/or challengin encounter for them with some RP viable elements there?

The answer would entirely depend on the world you have built.

The encounter builder on dndbeyond is a good place to try to build something suitable to fight against 4 x level 3 characters.

Make sure that they have to have several encounters before any short rest - this will stretch their resources. The first encounter might fall quickly, but hopefully it uses some of the party's resources to the point that the third or fourth encounter has them with much more limited options.

As Farling said, stretching resources is really important if you are going by the tables - otherwise they are going to be very easy for the group. 

As you wear down spell slots, HP and slap a few conditions on them, things start changing quite quickly. Using the environment can help too. 
Personally, I like making use of lower CR opponents that get smart about using their environment for cover, throwing stuff at characters, dropping trees on them or similar. But starting out with the encounter builder or the tables in xanathar's and then stack a few ones in at a time will work too. Combining different creatures in an encounter is also good from providing a more versatile threat so they don't just shore up against those goblin arrows.

There's a blog and book by Keith Ammann that's worth taking a look at too - it's called The Monsters know what they're doing" and provides some great ideas and reflections on different monsters

2nd what's been said thus far, especially the blog from Keith Ammann. Link here for your convenience - The Monsters Know.

I might suggest rolling through chapter 3 of the DMG, specifically the Creating a Combat Encounter. It's the explanation behind how the encounter builder works, and has a lot more inspiration for building encounters that might be overlooked if you rely solely on the encounter builder. The Encounter Builder is still a great tool, and I do advocate for its use. Here's another version should the first link not work. 

If you're looking for adventure ideas there used to be an article series here on DDB - Encounter of the Week penned by James Haeck that you might find useful. There are some good products on DMsGuild and DriveThru RPG that are free or pay what you want (PWYW).  

If your players are breezing through your combat then you might want to have a mystery or a monster that does not head on attack the party but tricks them first, in order to defeat them.

Good examples of more clever monsters that you could personalize as NPC's and villains (and that work for about level 3) are: Hags (evil witch-like beings), dopplegangers (shapeshifting tricksters).

Or, you could surprise your players with seemingly normal objects becoming 'alive', mimics and animated objects do this well.

Also - don't be afraid to sometimes throw away the table, and put in something really dangerous!! 
Monsters don't neatly queue up along a progression line to give the characters a fair fight. Sometimes they might pick up information or come across things that are either way easy or way powerful. As long as you , as a DM, provide them with opportunities for "outs" against a powerful enemy, it will be an exciting encounter and valuable lesson at the same time for the group. 

So first and foremost thank you very much (all of you ) for your time and effort spend on helping me . I did go through all of your advice and there is going to be mistery incorporated and some pre events before main one . main villain is going to be Spectator. with with warlock lvl 1 npc aide. and before that two animated armours with one dust imp and 1 or two steam imps with ofc time in between all of it and exploration beforehand and afterwards. 

one more time thank you very much for your advice

A Spectator with a level 1(CR1/4) assistant is going to be an extremely easy fight. There is a 56% chance the Spectator won't even launch a damage attack on any one turn, and the level 1 is probably going down in 1-2 attacks.

The Spectator also has only 39 hit points. I'd expect it to die on turn 2 of the combat.

Use the Encounter Builder tool on this site to help guide you. If you are going to have 3 or fewer fights during one adventuring day, each encounter should be Deadly difficulty because the difficulties are based around 6-8 encounters per day.

For comparison, I had 4 x level 2 PCs take out a Mindwitness after having fought 2 other encounters, one against 6 Zombies, and a second against 2 Zombies and a Zombie draft horse that had an AoE breath attack. They were able to get a Short Rest before the Mindwitness.

thx ill keep it in mind.

If you struggle with let's say, balancing, I'd advice checking up certain strats for low CR monsters, like 1-2 CR and then make a group encounter for them that revolves around said strat. This is a good way to get a feel for how well they do under... more intelligent combat scenarios and kinda gives you a few chances to prod at them to see how well they take it, then you can always up the difficulty later if they breeze through it.

In DnD stand alone monsters are hard to make difficult encounters unless you do some DM trickery, a dragon sure, that can be deadly AF, but a single werewolf for example is.. not very hard at all. Numbers is key, then strategy after.

I would go on the lighter side because let's say they fight some goblins and they're destroying the goblins without much difficulty. you can always just make 5 goblins step in to make the difficulty feel more balanced