So sorry to hear about your recurrent miscarriages.

I don't think your weight has anything to do with it as it sounds like your in a very healthy range. You do not have trouble conceiving at all. 3 pregnancies in 8months is a sign of very good fertility. In saying this as it has now been 3 losses I am sure there are some tests they could run to see if there is a reason for your miscarriages.

Has your GP offered you any preliminary tests?