Can a i3 3rd gen handle a modpack with 400+ mods with 10-15 players?

No probably not at all

What is the exact CPU and what modpack

I do not think so

Currently the laptop is dead, but i think its a 3220 and the pack is beyond depths, its more of a gpu based modpack

I will use Ubuntu/Ubuntu server

What budget CPU would you recommend? İ found a i5 6th gen for cheap too but i dont want to spend money much.

Sorry, what do you mean by GPU based modback

A lot of things are graphically intense, for example on my personal laptop that has i7 13th gen and a rtx 4060 i get 100% gpu usage meanwhile my cpu is Bing chilling with %30 usage while a lot of background programs like opera, razer synapse, discord etc. Open

This is wrong on so many levels

If the mod adds a new entire entity or block or biome and stuff, you need that on the server, but if it's just shader you actually shouldn't have it on the server, because it doesn't do anything on the server

Yes i know

The server doesn't use GPU, it doesn't matter that your mods are graphical it will still have the same performance impact

The modpack doesnt have much structures, and most of the entities spawn when a event is going on

Yes thats why i didnt specify my gpu either

What i wanted to say by gpu based is that beyond shaders, particle effects etc the modpack is rather plain

Particles effects are client side

I think you should start by finding out which mods are server side and which are client sided

Loading client sided mods on the server still has the effect of using more ram and increasing load time

And complicating the dependency tree

Yeah it is heavily dependant on mods but i just wanted to get a general idea. I will get another server PC if this one doesnt work out.

Also do you know how Can i stress test the server? I dont have many friends

no need to complicate things, the answer is that CPU will definitely not support that workload