3HP continuous, I'd guess, but I see a lot of 110v motors rated that high (probably at peak) or higher.
There are many different types of motors and horsepower numbers are computed/measured/exaggerated somewhat differently for each type. A 3HP router than runs on 120V is completely different than a 3HP tablesaw motor, which requires 220V...
If you compare specs for tablesaws, you'll see that you can't get much more than 1.5HP with 120V. Some will claim 1.75HP, but I tend to think they're exaggerating. To get more power, you have to go to up 220V. Once you max out 220V, you have to use 3-phase industrial power. Don't remember where the cross-over point is but somewhere in the 3-5HP range I'd think.