single phase 120V convert to 3 phase 208V
If you need 3-phase, 208V for just one 3-phase motor, you can buy a variable frequency drive (VFD) that will accept 120 V, 1-phase input and provide 208V, 3-phase output and control the output voltage and frequency to provide variable speed. It is simple in the sense that you just buy a single product and connect it. You should be able to easily select a VFD that is suitable for any load that is suitable for the motor. The VFD is likely to be reasonably easy to connect, set up and use. The product itself, is quite complex, but you don't really need to understand what is inside the product. VFDs of this type are available in power ratings up to about 2 kW, about the maximum that a 20 amp, 120 volt, single-phase circuit can supply.

There are other phase and voltage conversion products and custom-made systems that could supply more power and more diverse loads. Some of them are conceptually simple, but have various performance and usability limitations. Some need to be adjusted to suit the load. Some require performing a manual start-up procedure each time they are used.

A single-phase motor driving a 3-phase generator is a simple concept, but may need quite a bit of excess supply current capacity for starting.