I'm making a personal finance spreadsheet and I have a balance column like in a checkbook. I want cell in that column to add the two cells to the left of it, and the one above it. (Credit + Debit + Previous Balance) What would be the formula for this?

Hypothetically assuming you are putting the balance formula in cell E2:

=C2+D2+E1

If your debits are not entered as negative numbers, then change the formula to...
=C2-D2+E1
...so that you are actually subtracting your debits from the balance