what are three feedback mechanisms to modulate the greenhouse effect?

Answer 1

One is surface albedo -- the reflectivity of the planet's surface and atmosphere, and how much light it reflects. A planet with heavy greenhouse gasses often has a higher atmospheric albedo, which limits the amount of sunlight that gets through.

Atmosphere density and scattering effects are another self-regulating mechanism. Thicker atmospheres, with a lot of greenhouse gasses, again reflect more light at higher altitudes, preventing it from getting down to the surface.