No boxxers are NOT IS never have been at least until the modern post mount versions. If you offer up the #3 to the Boxxer and then put the wheel in you will see the pads will miss a significant chunk of the rotor which isnt safe
I'm at work so I can't compare but I have a C2#6, a C2#3 and some 4 pot #3's and #6's and I think the main difference is in the position of the top tab hole, on boxxers the sit flatter than IS and the actual caliper is an entirely different shape...
I had to get a new caliper half to allow me to fit mine BITD and I would suggest you look for a #6 caliper rather than try and bodge it!
I'm at work so I can't compare but I have a C2#6, a C2#3 and some 4 pot #3's and #6's and I think the main difference is in the position of the top tab hole, on boxxers the sit flatter than IS and the actual caliper is an entirely different shape...
I had to get a new caliper half to allow me to fit mine BITD and I would suggest you look for a #6 caliper rather than try and bodge it!