How can I set the last one item always to the left with crossAxisCount 2?

[RoxasLong]

When I have 3 items,B is shortter than A,then C will under B just like this:
AB
AB
AC
C
C
Our Boss don't like this..She want it this way:
AB
AB
A
C
C
C
Please,How can I do this,I don't want to lose my job T.T,Thank you~