ive completed all 3 first campaings but the other ones are in grey so i cant play them in restoration of erathia

Do you have check-marks over all of the completed campaigns? When you complete a campaign, you need to save your game (the game asks you if you want to save, click yes) and then continue playing from that save. This will mark the campaigns as complete. You need to have all 3 campaigns marked as complete to unlock the rest.

Unfortunately, it's quite common, especially for new players who are used to the way modern games work, to click "New Game" after finishing a campaign, which starts a completely new game and doesn't mark the previous campaign as finished.

As Nocturen mentioned, you should have saved and continued after finishing a campaign.

What you can do now is use cheatcodes to unlock the campaigns, so you don't have to replay the original ones from scratch.

Use these cheats to complete the 3 first campaigns fast and proceed. Don't forget to save after you complete one campaign and continue from there.

http://www.heroesofmightandmagic.com/heroes3/cheatcodes.shtml