Sorry I'm not the one to talk to about blood typing. There used to be a woman on the Maternal & Child forum who knew a lot about genetics and blood type inheritance, but I haven't seen her posts in a while.
I posted awhile back because i already used one of those prenatal paternity centers. Paid them $1300 (not money i had - on a credit card) and then found out they were a scam. They won't even answer my calls now. I believe i can get them to cover the blood typing for the baby since i am RH-, but the insurance won't cover any paternity testing costs, which i believe could run another $1000=$1300. I know blood typing won't always work. I was just hoping that in this situation it might since guy #1 and i are both type "O" and guy # 2 is type "A". There are only 2 possible blood types for the baby and each belongs to one potential father (as long as guy #2 can find out if he is AA blood type and not AO).
Definitely worth checking into. How much money are you talking about? Because the prenatal blood-test outfits (the ones that are reliable, that is) are ranging from $1,700 to $2,100. If it's more than that, I'd consider the blood test, if you absolutely have to know before the baby is born.
I don't think insurance will cover the DNA portion of the amnio and the paternity testing. I just can't afford to shell out all that money if it doesn't. I had heard that they will cover it if the doctor bills it as "maternal anxiety." However, i am not sure how true that is.
I'm sorry, but I know nothing about blood typing and its implications for assessing paternity. Why not just have a DNA test when you have the amnio? (They won't do an amnio for that reason, but before prenatal DNA blood tests became available recently, amnios were the only way to know in advance of the baby's birth.)