You're going to have to be clearer here, 3 pairs of terminals means 6 terminals. The assumption being that your signal goes into one terminal and out the other.
You say you wish to connect pairs of terminals together so your 3 throws would connect terminal 1&2 then 3&4 and finally 5&6 for example.
If that is what you desire, a double pole triple throw rotary switch would work if you short the two poles together. Rocker switches have only two positions plus maybe a center off so that would not be a reasonable choice.
Another possibility is that you want two signals (stereo audio perhaps) to go to one of 3 destinations. A rotary switch would connect the poles to throw 1 or 2 or 3 for both the left and right signals. This would still be a double pole triple throw rotary switch but the poles would not be shorted together.
You'll need a pulse width modulator to control the motor speed yet maintain good torque. There are ways to create such a modulator with an LM555 timer IC. You use it output to control a power FET.
FDB603AL is such a FET and I found this one on an otherwise dead motherboard. It is rated to handle 33A at 30V with a Vgs of 4.5 volts and having a Rds(on) of .036 ohms http://www.ortodoxism.ro/datasheets/fairchild/FDB603AL.pdf
I'll drop copy of a modulator that ran in Electronics Design onto alt.binaries.schematics.electronic
Full speed with switch down, slow speed with switch off, medium speed with switch up.
Realisticly, using resistors has problems. Reduces torque, and they need to dissipate a *lot* of heat at 4 amps (I^2R). Instead, use PWM - and then you can use a pot and get the widest possible range of speed available, avoid the heat problem, and have better torque.
If you absolutely cannot use a pot, then you can still get 3 speeds using the scheme above with PWM where the resistors select the pulse width. They won't get hot that way and can be much smaller wattage.