Twins turned into Triplets???

Hey everyone! Okay, so my husband and I were very surprised when we found out we were expecting twins! Last week at my appointment, another sac was discovered. Which shocked everyone b/c huh?????? I am 29 weeks and I have another doctors appointment, Monday to see a specialist. The sonogram tech as well as my doctor and others that were called in said they had never seen anything like it before. My doc. told me not to worry b/c the babies had strong heartbeats & it was not getting in the way of their development. The sac was b/w them , but behind them, if that makes sense & was moving in the flow of the babies. We were told very little, on if it was actually another baby or what it was, but we are really confused. Has anyone else experienced this or heard of this? If so please share.