They have said that they will reserve around 15% of the fuel capacity of a first stage for reusability operations. At the point they need to impart the Delta-V to return to base, they will thus be 85% empty. 

Thus the need for only 3 engines instead of the 9 main engines for the retro propulsion burn. 

I won't try to do the math, but they claim they have, and think it will work.

There was some discussion I did not entirely follow about whether 2 or 3 burns are required to return.  They need to cancel the velocity in the forward direction, and start heading back.  Then need to control entry back into the atmosphere in a survivable fashion. 

Remember they are lofted upwards and forwards when MECO-1 and stage separation occur.  They can ride the upward component and focus on the forward vector to cancel it out.

For Falcon Heavy it is trickier, since the middle core stage burns till much later and is quite a bit higher and faster at it's MECO. 

Some have speculated that launching out of Texas, and recovering in Florida might buy some margin on reusability operations.