Is he really a series regular? He is already credited as a guest star.

Ah ok, sorry my mistake.

Now is another problem, when you click on Tom Hopper and look at his movies and shows, you can see Merlin (17 episodes) - and when you click on that, you can see episodes from 3. seasons - but he didnt played in that and in Merlin he isnt add? It is some bug?

Yup, there is a bug with all credits that are incorrectly added as season regular. There's nothing I can personally do to fix it. It was reported to Travis a couple weeks ago, but he's a pretty busy guy.