Natural three-gendered species?

I'm trying to wrap my imagination around some "three-gendered" systems. Also, I have one or two that I would like to understand deeply enough to extrapolate its cultural effects.

The basic Earthly examples:

• Mammals with heat. I'm thinking the gender identity for a "woman" should be different in and out of heat.
• Beehive, or other social insect with a queen. The workers are particularly interesting. (I suspect their default gender identity is neither male nor female.)