Was thinking about this, have a 2 engine regular turboprop with a third electric engine for extra performance (imagine a DC3 like the Conroy Tri-Turbo-Three). The 3rd electric engine would be run from the turboprops' alternator/generator and solar, with minimal batteries for emergency use.
An alternate version has the 3rd engine run by steam, the turbine engines produce the exhaust heat to power it, and cold upper air temps will cool the steam quickly.
In either case the 3rd engine does not have to be as powerful, it's just there for performance/emergency. Does either sound feasible?