I have a 3yr old female tabby, who is normally very active and very affectionate, but recently has decided that sleep is all she wants. She is gaing a lot of wieght, but if you pet her she will accept she isn't agerresive. Should I be concerend for her?

Hmm... no one has answered this. I personally would prefer more information.

A 3 year old cat should be fairly active still. Sure, cats do sleep a lot, even young ones, but they should also have active periods. I have a two year old who sleeps a good chunk of the day but then goes through her hyper spurts wrecking havoc on the house. :-)

The weight gain could be from the inactivity, or could be another symptom of some problem going on. If it's from inactivity, feed her less.

I do find this a bit odd, but I'd be curious if she started acting like this suddenly, or if this was gradual. There are some cats who just aren't all that active, but it sounds like it was maybe a bit sudden? In which case, you may want to make a trip to the vet to see if something is wrong.