Those go in my compost tumbler. I cut up the banana peels, smash the eggs shells and throw in coffee grinds with the biodegradable coffee filter.
Do you mean those 3 ingredients only?
Can wait to hear the answers!

Allison

Well, in the first place, the stuff you have mentioned is not fertilizer, they're soil amendments. They make your soil more friable and entice earthworms to make your garden their home too. Egg shells contribute some calcium and banana peels contribute some potassium. Neither of these as well as coffee grounds are fertilizers per se, but are all necessary to help your garden look its best.

By the time such materials are broken down by soil bacteria and earthworms into a form that is usuable by plants, the plant will be ready for them.

Actually, by the time the egg shells break down, it will be time for your great great great grandchildren to graduate from college. :)

I put all three to mix with the soil as I am planting. They are ammendments, but I do not have a compost bin yet and I just add to the soil when I am planting. Fertilizing is a different story which I am trying to figure out as well.

Eggshells are limestone such as you might buy as bagged lime, but as Hoov says they are too coarse to dissolve in the soil moisture and will probably have no effect during your lifetime.

Banana peels are no richer in potassium than potato or cucumber peels. They should go in the compost along with similar vegetable trash, all of which is slightly nutritious.

Coffee grounds contain significant nitrogen. 2-3 cups could be counted as a feed, comparable to one cup of alfalfa or a spoonful of Miracle Gro. Coffee grounds don't have potassium, though.