Watts are watts are watts. Nothing you are going to do is going to change that., except, as others mentioned, to add more losses to the equation. But when it comes to motor loads, there is also an efficiency and power factor component to the equation. In that case, 3 phase motors tend to be slightly more efficient than 1 phase motors, but 1 phase motors tend to have better power factor (because of the capacitors). Both things affect the power consumption and it usually ends up favoring 1 phase motors when the power is 3-5HP or under.
Here's an example:
Baldor 3HP 1 phase 230V cap start,
High Efficiency motor, nameplate FLC is 11.2A, .98 pf, 85.5% efficient
Link
Power consumed then is 11.2 x 230 x .98 x .885 = 2.16kW
Baldor 3HP 3 phase 230V
High Efficiency motor, nameplate FLC is 8A, pf is .77, eficiency is 90.5%
Link
Power consumed is 8 x 230 x 1.732 x .77 x .905 = 2.20kW
So less than 2% difference betweeen the 2, favoring the 1 phase motor.
However, whatever you do to convert the 1 phase supply to 3 phase is also going to have efficiency losses associated with it. Rotary phase converters are notoriously inefficient as mentioned above; 40% losses at least, usually more. Static phase converters (electrical parts, but not electronic) are slightly better but not always compatible with certain loads, and you may not find out until it is too late. Electronic phase converters, such as VFDs are the most efficient at 95-97%, but are also the most expensive and complex: more to go wrong.
So at best, you will come out with slightly worse than a wash on the efficiency, i.e. you gain nothing in the 3 phase motor but lose in the phase conversion. It may be a tiny little bit better if the 1 phase motor you are replacing is not Energy Efficient and the 3 phase motor is, but now you have spent a bundle of money and decreased your "Mean Time Between Failures" by adding complexity, getting little in return.
