i am so sorry. i know how you feel right now. i had an ectopic rupture a few months ago, and my hcg levels didn't drop, but they just didn't double, so they told me i was either gonna mc or it was ectopic. they were right. it was the most horrible thing i have ever had to go through. an even when they tell you its gonna fail, you still hold out hope. it sounds like you will probably mc. i feel for you and know this is hard. there are a lot of women who have been through similar situations on these forums. it is a great support. and you can offer your support to others who are going through the same thing.
good luck to you. i am here if you wanna talk.
With a prog level of 3.7 you should have been instructed this would result in a failed pregnancy (and possibly an ectopic- a pregnancy outside of the uterus). Supplemental progesterone has not been shown to be effective when used in this fashion. Additionally, your hCG levels have not risen appropriately. So no, this will not be a viable pregnancy. You can discuss mgmt options with your ob/gyn.
One of my friends had a mc and she knew about it at 6 weeks and didnt start bleeding until 8 weeks. your hcg levels should definetely be rising. sorry!! good luck.