You can but that takes some work, re-routing wires. I don't know how many breakers you are talking about, but if it's just 2 or 4 you can and you have room in your panel you can use a breaker interlock switch.

The silver bar mounted on the breaker handles does not allow both breakers to be on at the same time.

To answer your question, yes, you can use a transfer switch. But from the picture you can see the advantages of using the interlock. No extra wiring, no subpanel to install. No more breakers to buy. Not many electrical distributors will stock this, and the brand does matter. These usually don't work with breakers / panels from different brands. They are not permanent so they can be removed easily.

EDIT - This is why all information would be great to have. A xfer switch is made to move power from 1 panel to another panel. You can't switch some of the breakers of the panel. So basically the way I see it is that you will need two sub panels, 1 for one task and another for another task. Probably a couple of j-boxes to do some of the wire make-up because some of the wires won't be long enough. Lets say that totals to 30 or 50 amps (just guessing because you have not told anyone that.) Then instead of buying the more expensive xfer switch you can use the interlock, switching the breakers feeding each sub panel.

It really would help if you gave more info, like how many circuits (poles and amps) for one task and the same for the second task.

There are other items you can use for simple things, like breaker handle locks that screw on;

or breaker lock outs.

Either can be used to lock on or off. The breaker will still trip even when locked. I would prefer the one without the padlock.

You should also check local code to make sure there is no problem having the receptacles out of use, even if it is for part time.

Think I'll chime in here and add my two cents to these other good answers. There are two components to the original question:

1. How much less power does will the 3 phase A/C compressor unit use?
2. Longevity of three-phase motor versus single phase

Power

Motor Efficiency = Power Output / Power Input

It takes a certain amount of power to run the compressor regardless of single or three phase power... this is the power output. The power input is what you are trying to minimize, thus you want to increase efficiency as much as possible. The trouble is this metric (efficiency) is a hidden combination the components inside the compressor unit (compressor, fans). It is not necessarily true (but it usually is true) that three phase is more efficient than single phase.

I would look at the SEER rating which 'automatically' includes the motor efficiency for a better indicator of "cost to cool".

Longevity and Reliability

Three-phase motors and compressors are generally more reliable than their single-phase couterparts. But like anything else, there is more to it than that one attribute. Using Philps' automotive engine example, a 4 cylinder Honda might be more reliable that that old smallblock V8.

Vibration

Certainly less in the three-phase motor. And generally less vibration equals greater reliability.

Motor Starting

Three-phase motors have lots of starting torque and don't (typically) require any special circuitry to start (capacitors, centrifugal switches). And of course fewer components means fewer things to go wrong.

Cost

Is disappointing that the three-phase equipment is more expensive. Almost always, three-phase motors are actually cheaper than an equivalent single-phase motor.