Three XM-Ls is 10.05V. Add 0.2 for the driver and you still can run at full output power when the engine is off, and until the battery is pretty dead. This configuration puts lots more stress on the driver.
Three SST-x0 in series is something like 11.4V, so it would probably run on full until the battery is somewhat discharged.
Any of these configurations would be the 'preferred' choice, even though they aren't all 'optimum'.