Part to of where do Babies come from. Tigress showed Po.

From the Previous pics there were plenty of Questions of who the Father is.

This is my Theory.

Po is outta the question because this scenario would never have happened.

On the Bridge scene when the Furious 5 combatted Tai Lung he did a move where he incapacitated all of them.
In fact with that Technique he could have had his way with Tigress easy (dragging her behind the bushes, doing his thing) and then healed Crane so that he could take them away.
She kept it secret out of shame untill the swelling started.

The Snow leopard has the Highest chance of fathering the cubs of Master Tigress...

.This is my Theory at least.