Your 2017 Form W-2 showing $7,750 in box 12 is not a problem. When entering this into 2017 TurboTax, TurboTax will ask if any of the amount is for the prior year. When going through the HSA section of TurboTax, answer Yes when TurboTax asks if your employer told you about any other contributions. Indicate that your employer told you that $3,350 was a contribution made in 2017 for 2016.
Similarly, the $3,350 should have been reported on your 2016 Form 8889 as an employer contribution. This would have been done by indicating to this same question in 2016 TurboTax your employer made a contributions of $3,350 in 2017 for 2016.
(Note that any amount that you personally deposit in the HSA rather than being deposited through your employer will not appear as part of the amount reported with code W in box 12 of your W-2.)
