Some people (myself included) like to approach this question using the “chipload” value, that’s the thickness of the chips you are getting when cutting with a given combination of RPM, feedrate, and endmill.
The idea is that for a given material and tool, there is a sweet spot for the chipload value that can be reached using a variety of RPM/Feedrate/endmills combination.
Chipload in inches = (Feedrate in inches per min) / (Number of flutes x RPM)
In your case, the chipload value that seems to grant good results is:
chipload = 20ipm / (2flutes x 10.000RPM) = 0.001"
(which does not surprise me as 0.001" is kind of the magical chipload value for cutting aluminium, and more)
The chipload value does not depend on the endmill diameter, so no need to compensate for that (however, the MAXIMUM chipload that a given endmill can take does depend on its diameter). In your case, if a 3.175mm endmill could take it, a 3mm endmill will take it just fine.
So since you are going from 2 flute to 3 flute, the only thing to do to keep the same sweet chipload of 0.001", is to adjust the feedrate:
new_feedrate = chipload x RPM x nb_flutes = 0.001" x 10.000 x 3 = 30ipm