Cross Reference In Form

I have 3 fields in a form all have binding to the same table.
the problem i am facing is as follows

when i enter two fields in the form the third should calculate on its own.
Lets say the three fields are names field1, field2, and field3

scenario A
I enter field1 and field2. field3 should calculate on its own by adding field1 & field2

scenario B
I enter field2 and field3. field1 should calculate in its own by subtracting field2 from field3.

any time by changing any of the field1 or field3 the other one should change based on field2.

if field2 is changed then field3 should change.

i hope i have explained the problem in detail..

please help

thanks

aseem