I would consider as mentioned, not focus on what might happen, plan your piping and wire sizes, ATS etc. and If the option to upgrade the generator to a three phase does come, your initial install for the single phase generator will be a lot easier to manage and not add doubt which helps convince the inspectors if applicable.
Your initial install of conduit and wiring will be more than enough in size and capacity, to enable a smooth 3 phase gen/panel install, should they decide to upgrade.
You can always add additional wires or boxes now if you know or have ownership in this current and future process and consider that when your sizing the boxes and your breaker AIC ratings, pad sizing and grounding.
Plan on future use of the single phase generator and panel if they decide to upgrade.
Typical emergency lighting circuits for small businesses or large, includes some type of interim battery back up so there's never a loss of the emergency lighting.
By the time they decide to upgrade, regardless if the three phase infrastructure as mentioned above has access to multiple taps for voltage requirements today or is an option, I would not personally install it that way as, you put yourself at risk financially if it has issues and adding a potential load imbalance to the mix.
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