You should be happy for what you have! Cherish the children you have now. If you ever really want to have a third and it's possible AND safe to get off the medication for an extended time you could try. Just don't put your life in danger just to have another child, the children you have now need their mother.
This is really personal too, but I'm 23 and may never have children. My fiance went through chemotherapy for several months and a couple surgeries. The first surgery was to remove the testicle with cancer and the second was to remove all the lymph nodes in his stomach/groin area (15-20 that sit directly on the reproductive nerves). The chemo, and the lymph node surgery lowered his chance of getting me pregnant the natural way. We have enough in the bank for three in vitros, but I also have some endometriosis that might totally ruin it for us in the endI'm not religious, but I would really rather not do the in vitro and I definitely won't use another man's sperm. If it's not meant to be, it's not meant to be.
Also, if you don't want an IUD, I love love love depo, but it is a little more maintenance than an IUD.