Hi, Deb-
Let's say the budget column is A, the first revision column is B, the
second revision column is C; actuals YTD are in D, and the Balance
formula is in E. Let's assume the revisions are blank until they are
updated, since it's possible for a budget to be revised to zero.
(Trying to avoid a logic flaw, here.)

Try this formula:
=IF(C4<"",C4-D4,IF(B4<"",B4-D4,A4-D4))

It works from right to left to find the first non-blank entry starting
with revision 2; if revision 2 is blank it checks for a revision 1; if
rev 1 is blank it performs the math on the original budget amount.

Will that get you where you need to go?

Dave O