If i was going down the road of electrics i would go this way. a 3hp motor will cost you alot as it produces alot of power in a single application that is why they are priced so high. The way i would do it is use 2 lower power motors to produce the power you need.
These motors are designed to give extra power to scooters so 2 of these would produce the power you would need. You would want to have two separate axles, going on the idea of most robots.
http://www.monsterscooterparts.com/36vo10mo.html
You can then use these speed controller to control the speed they spin to have both going at the same rpm
http://www.evparts.com/prod-CT5323.htm
If he do that (assuming that both motors would spin at the same speed - which NEVER happens, due to a small differences in motors) , he will get in trouble when cornering (outer drive wheel should spin faster then inner wheel - so he will end up in a hedge). Type of the controller that you talking about is really hi tech and sophisticated (way out Jammer budget). It could be done by some sensors on the steering wheel (to switch off inner wheel drive, when cornering) , but it's kind of complicated (not for those with know how).
@Jammer
You didn't read what I posted well.
Power (W or KW) = Amps * Volts .That means, if you have 2 KW motor (2000 Watts) and run it on 48 Volts , you'll be sucking up (that is your motor via controller) 2000 W / 48 V = 41.666 Amps (your controller should be able to withstand 50 Amps); if you run on 36 Volts , the math says that your motor will use 55.555 Amps , so you go with controller rated at 60 Amps (always higher).
It's important for you to understand that Volts means speed (more rpm) and the Amps means power/torque. Torque is the thing that moves vehicle (go kart) around. Power just means what it's capable of (that is the work done by system in a unit of time - I bet you like it so far - physics
). If you're satisfied with speed , the only way to ramp up the power (acceleration) is to let more Amps trough motor (and , therefore , trough controller). So you'll need a stronger controller (that means it can take more Amps).
Controllers are rated in Amps and Volts (that is simplifying for you to understand), so when you want to order one , and there (on the site) is rating in Watts , you should call the people that sells the stuff and ask for the Amp rating (if you know what voltage is - you can calculate it yourself by using above formula).
Now , HP rating vs KW rating isn't straight forward to deal with. In internal combustion engine the efficiency of the motor is around 20 - 25 % (in modern cars up to 28 %). The efficiency of electric motor is 85 % upwards (generalization - it depends of type and construction of motor). Roughly , if you deal 85% by 20% , you get 4.25:1 ratio in efficiency , between electric and internal combustion motor - you can round it to 4:1 ratio. 1HP = 0.75KW so 3HP = 3*0.75KW = 2.25 KW. Remember , the electric motor is about 4 times more efficient so 2.25 / 4 = 0.5625 KW , which is
~562,5 W.
That is what you need if you want the equivalent of 3HP. So , to avoid some glitches (imperfections in our calculation) and system loses (mechanical and other),
you'll be good at 1000 W (1 KW) motor. You can buy a 36 V motor , cause you can over volt it without any problems twice that voltage. Only consideration is current (Amps). Ratings of the motor and controller , that I've wrote of, are continuous ratings , they peak much more (e.g. when you starting go kart from a stand still) but that shouldn't be your concern , cause manufacturers count on that when designing the stuff.
For 24 Volts system (batteries) , you'll need 24V / ~40 Amps controller , for 36 Volts - controller is 36 V / ~30 Amps , for 48 Volts - controller is 48V / ~20 Amps. If I were you , I would go with controller with rating of 40 - 50 Amps (no matter what the voltage is). 36 Volt system is ok , better 48 Volt system(less strain on the batteries and plenty of oomph to go arround and some more range).
Shop around (
I've said it before : Google) and see what can be obtained on E bay or , generally , on the net.
Hope that this cleared some things and helps.
.