Choumajuu! ~Seieki Juuten Dungeon Kouryaku~

The protagonist, who can't use good magic but has magical power, and the two heroines, who can use powerful magic but run out of magical power quickly, are in a magic school story where they bet their graduation. The protagonist can share magical power with others by engaging in sexual activities... so of course, the main activity is sex! They may not be useful in battles against monsters, but the three of them will combine their powers and bodies to overcome various (erotic) difficulties!