I hear a third HB

Lisa
Hey ladies!I've been hearing a HB since 8 weeks with my at home Doppler. At a 12 week scan we saw two babies!!! I'm 14-5 now and can hear both HBs. One on the left and another on the right. The left (Baby B) is consistently and noticeably faster at 150-160, than the right (Baby A).
 However!, on the right I hear Baby A's HB about 130-140 and it sounds like in the background another HB that's slower. Is that the placenta or my own HB? 
This was the first time I've heard more than one HB at the same time. I can only catch each baby's HB on distinct sides which is why I'm wondering what I'm hearing. 

Thanks! I tried to google and didn't come up with much.

(I'm also posting in General Pregnancy too)