I plugged in your numbers into G-Wizard. For the .75" 3-insert face mill the aggressive setting with 100% axial engagement:
4000 RPM DOC-.19" feed-75 MRR-10.67 ci/min
However, that yields a HP rating of 2.24, and I think it's unlikely that you'd really get that from the 3HP motor. Reducing the feed rate to 40 reduces the HP requirement to 1.2 and the MRR to 5.69.
For the 1/2" 3-flute endmill the aggressive numbers are:
3522 RPM, DOC-1.36 feed 14.5 MRR 9.83 HP-2
If you reduce the DOC to .25 then:
4000 RPM feed-38 MRR 4.8 HP-1
Conservative setting has feed-16 MRR-2 HP .42
One of the things G-Wizard brings out is that a larger DOC with slower feeds give a faster MRR. Face mills are often limited to shallow DOC depending on the insert geometry.
Assuming that that 1/2" carbide endmill is expensive, you might start out with the conservative feed and increase gradually until it complains. The G-Wizard settings limit the deflection is less than .001" at the tip, which is the danger point for breakage.
All the calcs I did above assumed full width cuts. For pocketing and 60% axial engagement the feeds would increase.
With the .5" 3-flute at 1.5 DOC and 62% engagement:
2888 RPM feed-16 MRR-7.8 HP-1.6 (aggressive setting)
2475 RPM feed-6 MRR-2.8 HP-.58 (conservative)
So full depth pocketing should work at somewhere in that range.