Bombardier Inc.’s new commercial aircraft boss said Thursday that it is considering of a third model of its new CSeries jetliner that could compete with the core single-aisle jet offerings from its European and American rivals.

Bombardier’s current CSeries jets compete with the smallest models from Boeing Co. and Airbus Group NV. The Canadian company has struggled with slow sales, delays, and high costs for developing those models. A third model would compete in the huge market for single-aisle jets with roughly 160 to 180...