I have a table with 3 fields (columns). I have a form with 3 fields bound to the 3 fields in the table. I want field C calculate field B minus field C and return this value to the table. By using an expression as the control source I am able to make the calculation on the form but I am taking away the binding to the field in the table. How can I calculate this and still retain the binding to the field?

For starters, I wouldn't suggest having a field that is just calculated - you can just run a query that will automatically. With a query such as:

If you want it displayed on the form, you can just have an unbound textbox with the control source being something along the lines of:

Storing the difference of the two would just take up space.

HOWEVER, having said that
If, for some strange reason you REALLY REALLY need to have it stored, you could put some code into the form's BeforeUpdate event to calcutate the difference, or on some other event, such as the OnEnter event of the field.

HTH

Steven