Cells A1, A2, and A3 are provided for users to enter text
describing an action. A1 is always filled in, and A2 and
A3 may, or may not, be blank.

In cells B1, B2, and B3 users enter a percentage value
associated with the "A" value. Therefore, the values in
the "B" cells must sum to 100%.

Three possibilities:
A B
1 Do this action 100%

1 Do this action 75%
2 Do something different 25%

1 Do this action 65%
2 Do something different 25%
3 Do yet another thing 10%

Can someone help me with code and messages to force this
rigor?

Thanks,
Jerry