Whereabouts are you located? The Letterkenny exchange seems to be having some speed issues for many customers. Also, the 3Mb speed has in some cases shown up the poor quality of the eircom copper. Your line may have been borderline 2Mb, but now that you're trying to force 3Mb down it, the line may just be unable to take it and it's dropping down to a lower speed.
You may also want to check out their Support Forums:
http://support.smarttelecom.ie/forums/index.php
Many of the support staff are Admins on this site and they are (generally) fairly quick in giving you a response and hopefully resolving the problem!