Which serwer from digitalocean should I choose for new forum (3000 users)?
Start with the $10/months and upgrade or downgrade if need be.
It’s not so much the number of users, but what they are doing (posting, registering, searching etc), and when they are doing that will influence what you need. Take your best guess (what Regis suggested seems sound) and monitor the workload on the server using the tools built into discourse, or even add a web-based performance tools like monit etc.
Are you expecting to hit 3000 users out of the gate, or are you migrating from another platform?
I’ve a +3000 users on my forum and everything is fine with the 10$/month plan of DigitalOcean
I think OVH’s first plan will be enough for you.