Freaking out!!!! This can't be? Or can it? Idk

So not only did I find out that I was having mono/di twins boys at my 20 week ultrasound but today at my stress test they nurse and I saw 3 hearts! I'm 33 weeks and I can't fathom to think that they missed a baby this whole time but then again they missed the twins for half of the pregnancy. I can't stop thinking about it. The nurse and I looked at eachother when we found the third one and it was completely on the other side no where near the other 2 hearts. We saw the third heart twice but she said the baby must've moved because "there's no way the doctors missed it all these times" but then again we didn't find out about twins until 20weeks!! Half way!! So yea I'm freaking out ugh... riding it out until my next scan to make sure how many there are in there