You should enter your Jan-Mar 2018 RRSP contributions in your 2017 tax return information (first 60 days of 2018) and indicate in the RRSP section that you do not want to claim this amount in 2017 and you wish to carry forward this RRSP claim amount to your 2018 tax return (you will be provided this option when you are filling in the RRSP information).  The TurboTax program will carry forward this amount to your 2018 return next year and you will have this amount available for you to deduct in your 2018 return.  Because you made this contribution in 2018, you will NOT have an over-contribution of RRSPs in your 2017 tax return.