Simplest might be to have an invisible calculated control txtSum and update it with AfterUpdate for each checkbox: txtSum = txt1+txt2+txt3 while Balance = Amount-txtSum.
However, the design seems wrong anyway. If you had to add a 4th cheque you have much re-design to do that should not be necessary. Even if you will never have a 4th cheque, the litmus test is "if you did would a redesign be necessary" and since the answer is yes, you're on the wrong path.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.