After many weeks, I finally discovered the problems.
The first problem was that when I removed the 3rd dependent, all of his college-related expenses and information were deleted...so I had to re-enter them.
The other problem which caused a much bigger change in my tax bill was my filling status. For some reason, when I had removed the 3rd dependent, TurboTax changed my filing status from MFJ to MFS. Once, I figured that out and changed my filling status back to MFJ for both my Federal and State returns, everything went back to the original values.
