It went very well and he did almost the whole thing by himself. I stood by and read the recipe to him, cracked the eggs, helped measure out the oil and vanilla (both were very full bottles), and put it in the oven. Otherwise, he did it all himself. He probably could have done more but I was very paranoid about having a brand new bottle of vegetable oil spilled everywhere and the eggs he cracks for breakfast frequently have shells in them. With a little practice I think he would easily be able to make the entire cake by himself. It's a great recipe for this type of thing because it's very forgiving to the imprecise measurements of a three year old and it's very basic so it would be easy to fancy it up with lots of different flavorings and add ins.
We kept it simple and just stirred in some chocolate chips (and I only had him use one scoop of sugar to keep it less sweet). He liked and so did I so we (wait - HE) will be making it again for sure! You can find a copy of the recipe here.
I even had him clean up the (rather large) mess he made too!