bucknast:
Hey all, I'm new to arduino......start, stop, and control the speed of 3 x 3-ph 1/2 HP 3600 RPM AC motors.
Can anyone recommend...
Yep, I strongly recommend that you don't try this yourself. 1/2 HP is not an 'exercise for the student.'
Not that Arduino can't do it perfectly well, but to have -ANY- chance of success on the first try requires 'very familiar, at ease, and capable' skills.
Worst cases include pure HCF (Halt - Catch Fire,) or ripping whatever assembly a motor is mounted on to shreds and flinging shrapnel in many random directions. 
Sure there are sample circuits and tutorials, even newbie level stuff, but those samples do not include all the safety related bells and whistles that these kinds of voltage circuits and power assemblies require. -I- understand that -you- understand how to design and build a typical physical assembly, but there's only a very limited -range- of electrical, electronic, and software design that will -prevent- that physical assembly from destroying itself if there's even a minor error/bug in the electrical/electronic/software.
Shortest answer: if you don't already know how to do it don't try it. A few hours of (licensed/insured) electrical/electronic engineer consult time can produce something totally doable that you -can- easily build.