Damsels are very aggressive and territorial. If you get them now, you won't be able to add other fish later. As far as adding a 3rd to the two you have now, it's been my experience that it does not take long for fish to establish territory so depending on the size of your tank, the two may reject the new one. I would just watch them carefully (add the 3rd when you can be there to watch & rescue if there is a problem.)