say you have fields f1, f2, f3 -
F3 can be completed only if f1 AND f2 are completed.
all three are choice fields with pending or completed as valid values.
got to list settings - then in 'item validation' - put the following
=OR( AND(OR([@F1]="Pending", [@F2]="Pending"), [@F3]="Pending"), AND([@F1]="Complete", [@F2]="Complete", [@F3]="Complete") )
this is directly from excel (the best way to experiment with these things).
Plreace @F1 with F1, etc.
Note - this returns false if the two are pending and f3 is not complete but that would be easy to fix.
This returns the following (f1, f2, f3
Pending Pending Pending TRUE
Pending Complete Pending TRUE
Complete Pending Pending TRUE
Complete Complete Pending FALSE