I currently have 2- 1 year olds and 2- 2 year olds and personally love this group, the 2- 2year olds are doing the same things at the same time (potty training, learning to talk, playing independently ect..) and the 2- 1 year olds are also the same I find it easier to have kids around the same age group that way they are interested in the same things and your not having to do all these different activities geared to 3 different age groups. The babies are exploring (Crawling around and playing with toys and exploring their surroundings) where as the 2 year olds I can bake cupcakes with and do crafts ect. So all that being said if I were you I would try for another 3 year old since 1 year olds don't care much about having someone to play with and it is likely the 3 year olds will play with the 1- year old anyways....3 year olds love to have a companion and need attention if you have 2- 1 year olds the 3 year old would most likely need a lot of attention for you rather then taking the pressure off of you by having another little one to play with them.
Hope that helps