You might want to post this in Electrical wiring. There is more activity there and the practical knowledge might be better there as well.

The short answer is yes. 3 hp translates to about 2240 watts. A generously -sized alternator from a large car can run up to 2500 watts.

The details are more involved. There are lots of different automotive generators. They vary in size and in the rpm required for peak output. Some get higher output at lower RPM and that is what you want. If you dig, you should be able to get output vs. rpm curves for some alternators. That will help you decide if you want 1:1 drive. I expect you will want the alternator at a higher rpm than 3000. Note that in some newer vehicles the alternators are controlled from the main computer rather than an integral or separate voltage regulator.

Here are a couple of examples you can see if page down. Note that the 2004 alternator is controlled by the vehicle computer so you can't easily use that.

http://www.pantherfreedom.net/techlib/article22.html

An alternator designed for ambulance or other similar use might give you higher output and at low RPM. You might be best off with the engine being the limiting factor. If you overload the genet, the engine will bog down. If the alternator is limiting, the engine might spin happily while the alternator burns up.

Here is an idea. Call some automotive electrical specialist shops and see if they can fix you up. If you can find one that likes to do custom applications rather than just fix things up the way they were, you will be in luck.