IF DATE Multiple conditions

Hello,

I need help with rewriting this code:
=IF(A1<DATE(2015,12,31), "3", IF(A1>DATE(2017,12,31),"2", IF(A1>DATE(2020,12,31), "1")))

I'm trying to categorize those date cutoffs into 3 as such:
2015 is 3
2017 is 2
2020 is 1

but apparently, the last category does not go into effect because of the first 2 conditions. So everything after date 2020 is still categorize as 2.

Does anyone know a way to categorize these date cutoffs into 3?

Thanks in advance!