Refund for multiple years not filed yet

[pippin0]

I am a Canadian citizen who recieves an IRA payment yearly and the fund withholds 30% tax instead of the correct 15%. I must get an ITIN in order to file as a Canadian and I have 3 years to claim back. Can I file multiple years at once or do I have to make 3 separate tax returns. I will be claiming for 2019, 2020 and 2021.

[Critter-3]

All 3 years must be filed separately so start with the 2019 return ... mail it in with  the W-7 for the ITIN.  Once the ITIN is issued then complete and mail in the 2020 return  and efile the 2021 return.  Be aware that the ITIN could take several months to process but you cannot file any other returns without it.   Then give the ITIN to the company doing the withholding so it can be done correctly ... they had to withhold at 30% because you did not have an ITIN on file.