The difference between K-1 and K-3 is that whether you have married already or not. Even you have baby together, if you are not legally married, you have to apply K-1. If you are married already, you can apply K-3. The baby will be under K-2 or K-4.
However, The issue here is the timing. If you apply now, the baby is not born yet so you can not apply for him/her. But by the time of getting the visa, the baby will be born, which means he/she needs a visa to come into the conuntry. I do not know about the answer of the issue. May I suggest your hu*****and contact with a lawer.
Do not worry, even it is slow, you will be together finally. So please be happy for the baby.