It is usually the other way around... most RE's don't like to transfer more than one 5-day blast as they have an extremely high chance of implanting. Day 3 embryo's are not as advanced as the 5-day and it is more of an educated guess if they have picked the strongest so they are more apt to put more back. I actually transferred 3x 3-day embryo's and I am pregnant now with a singleton. In my case, my embryo's never make it to 5-day blasts outside of me... but every woman is different.. good luck!!!