I entered three 1099-R forms and it's adding them up for an inaccurate total gross distribution. What am I doing wrong?

If you received three 1099-R's, you would enter them all in TurboTax.  The Code in Box 7 on the form, plus the follow-up questions determine taxability.

The totals show on Form 1040 in Lines 4a and 5a, but the taxable amounts are shown on 4b and 5b.

If you just entered one 1099-R multiple times, type '1099-R' in the Search area, then 'Jump to 1099-r'.  At the summary page, you can delete the duplicate entries. 

Here's more info on How to Enter Form 1099-R. 

Why did you get 3  forms?  Did you rollover or convert some to another account before you took the distribution?  Don't worry about the gross amount on 1040 line 4a or 5a. That's just the total of all the 1099R forms even for duplicate amounts.   It's the taxable amount on line 4b or 5b that's important.  If you moved some to another account or converted to a ROTH it will say Rollover by line b.