Can I use a 3-wire+ground to serve a 240v and a 120v circuit?
Not in this case, because you can't put lights or outlets on a 30A circuit.
Note that also, you can't have receptacles on a circuit where more than 50% of ampacity is needed for hardwired loads.
So it could happen if, say, the numbers worked out (20A circuit, 240V/7A mini-split, then yeah, you could use both legs for 120V stuff).

Honestly, the 10/3 cable was a blunder.  Water heaters don't need 3-wire feeds.   You could, I suppose, fit a subpanel there, with a 30A breaker for water heater and a 15/20A outlet for lights -- however, you'd have to do a load calculation for outlets, because a water heater uses most of the feeder ampacity.
But for a subpanel feeder, for the same price as 10/3, you can get 2-2-2-4 Al feeder which is good for 90 amps instead of 30.