Probably because she is the only one producing the pheramones that say "I'm cheap, I'm easy, one beer and I'm all yours." Okay, kidding. But she is probably the only one in the tank producing the pheramones that say she is ready to mate. The other two may already be pregnate, and therefore are not receptive to the male and are not producing pheramones that attract the male. He may try to advance on them if he runs out of receptive ladies, but they will probably either flee him, or tear him to little bits.
~~Colesea
