Interest Relief for Rental Properties

[helllohello]

I have a house rented on HAP since 2016. I didnt fill out the form - Declaration of Undertaking for Deduction of Interest on Borrowings to the Private Residential Tenancies Board (PRTB) . The deadline for submitting the Declaration in relation to leases entered into before 1 January 2016 was 31 March 2016.
Can anyone tell me if I cant reclaim this interest now at the end of the 3 years ?