Set up a text field with an auto-enter calculation that combines the three fields such as:
FieldA & "|" & FieldB & "|" & FieldC
Don't separate the values with returns.
Update this field in existing records using Replace Field Contents or the trick described here: Updating values in auto-enter calc fields without using Replace Field Contents
Now set up a unique values validation on this text field.
There are also several other ways to check to see if a record with a particular combination of values exists.
a) a script could perform a find to find any records with the same three values
b) A relationship that matches records using these three fields as match fields could match to a second occurrence of the same table
c) ExecuteSQL can be used to query your table for any existing records with the same three values.
Thanks, I can implement this without craziness.