As RC says, 3x inverters in "Y" has a neutral. You have either 120/208Y or 230/400Y
In the case of 120/208Y, an auto-transformer can take 120V L-N and make 120/240V L-N-L split-phase
In the case of 230/400Y, you could use a transformer with three identical (or similar) windings to get 115/230V with an extra 115V hanging off one leg. But more likely use an isolation transformer.
Isolation transformer, could take in either 230V or 400V, the latter using two inverter outputs but drawing current at a phase angle.
I have tried 120V to 120V isolation transformers, using L1 as one phase of 120/240V split-phase, and taking a 120V "vector" fed by L2 into transformer primary, isolation with secondary wining making a 120V "vector" off L3. That makes something like a split-phase 120/240V in line with L1. But it is off a bit in voltage and phase depending on load. It might work for what you want to drive, but it didn't behave nice to synch my 3-phase inverters with single-phase grid.
If you have 3-phase delta, e.g. 240V delta with no ground reference, a "zig-zag" transformer is common way to generate neutral.
Alternative is "T"; one transformer with two identical 1:1 windings for 120V & 120V generates midpoint between two legs say L2 and L3. Then a second transformer 2:1 for 138V & 69V (approx?) between L1 and that midpoint generates neutral.
Showing a work-in-progress assembling 3-phase SMA system. 3x SI-5048-US are configured for 3-phase and produce 120/208Y, rated 15 kW continuous. Sunny TriPower 30000TL-US-10 is 480V delta (or 277/480Y), rated 30 kW. TriPower combiner box will be used, but for now just one string being connected...
diysolarforum.com
Most transformers aren't as nice to drive with inverter as you might expect.
So you think you can backfeed a transformer secondary, either as step-up or as auto-transformer? Turns out these things aren't so ideal and reversible as one might expect. It isn't just the greater inrush (primary is often wound outside secondary to reduce inrush current), but also idle current...
diysolarforum.com
You can use transformers with 240/480V primary, 120/240V secondary to make some of these configurations. It works best if "240V" primary windings are driven with 120V, and then two "120V" secondary windings connected in series for "240V" makes 120V. Or, drive one "240V" primary winding with 120V, and second "240V" primary winding makes 120V.
But anything 3-phase prefers perfect symmetry, which is lost when one phase is generated from another. Delta vs. WYE connection of transformers and motors may affect how much excess current gets drawn. Just for transformers, I'm getting nicer behavior with three isolated transformers rather than a single 3-phase E-core transformer.