I can't find a #3 16DP 20 PA on McMaster Carr. I did find a British maker that sells them for about $225 CAD tho!
I have 16DP #2, 6, & 8.
#2 might be close enough.
But they are all 14.5 not 20.
You are welcome to try them.
Did you try one of the outfits that make custom cutters?