I did not see KF2 so.... Spoiler

That was one of those truly heart rendering moments in the second movie. It's described in this wikia page:

http://kungfupanda.wikia.com/wiki/Po's_mother

They replay a portion of the scene in KFP3 as you see Po's mom hiding a baby Po in a cabbage cart and then distracting the army of wolves by running away in front of them so that they will chase her and not find her baby. And then she is cut down by bullets.

By the way, there are two two-minute videos that recap the previous Kung Fu Pandas, enough to give the gist of the story, but without some of the details like the one you asked about. They were put out by Dreamworks TV. They are:

Kung Fu Panda, two-minute recap: https://www.youtube.com/watch?v=alfdf27jb0M

and

Kung Fu Panda 2, two-minute recap: https://www.youtube.com/watch?v=dMOUAEaP5zM

Thanks.(PS This movie makes me want to order chinese take out.:-))

HA HA HA