Trick them

Kasha knew she was outnumbered, and it'd be easy for the succubi to corner her and do what they wanted with her. After all, they could all pass if they made the mortal cum. But what if there was a way to turn them against each other?

"Oh you ladies are all so sexy," Kasha said, "But I can't decide which of you to fuck first~"

"Me of course!" each of the three succubi answered, immediately turning on each other. "She's mine!"

Any thoughts of passing together while teaming up against the mortal were gone.

One of the succubi began to unbutton her shirt, flashing her cleavage to the others. One licked her lips, revealing an alarmingly long tongue. The third saw the first begin to unbutton and made her move, reaching down to stroke the succubi's cock while her hands were occupied.

The succubi focused on her tits tried to react, but the one with the long tongue saw the moment of vulnerability and pounced, grabbing the other by the wrists and then forcing her long tongue down the captive succubus' mouth.

With one succubi using her hands to stroke her cock and the other making out with her, the pinned succubus quickly came, spraying a jet of white into the air.

With one down, the other two shifted their attention to each other.

But Kasha simply turned to the succubus in the lab coat. "I've passed, correct?" she asked in a low voice.

The teacher smirked mischievously. "I think you've taught them a valuable lesson. And I'll make sure it sticks with some... supplementary lessons." Her cock, already hard, found a way to get even stiffer as she said that. "Congratulations, you may move on."