How many medium eggs should I use if a cake recipe calls for 3 large eggs?

You should use 4 medium in place of the large eggs. Just adding water to make up the difference really won't be quite accurate. A large egg would be 1/4 cup. So you would need to have enough eggs for 3/4 cup. If you get a little more it wont hurt the recipe. Less may make it not rise or hold together correctly.

4

Most recipes call for "large" eggs. As far as I can tell, store bought eggs basically come in one size. Unless these happen to be quite small for eggs, just use 3 of what you have. It won't really make a difference.

I have never paid any attention to large or med etc. If it says 3 I use 3. I buy Pillsbury Plus cake mixes the one with old and my cakes are so moist people love them. Never ever had a problem. The only thing I always follow is time they say to mix. I have found that really makes them ligher, fluffier. If it says mix 3 min. I time it and mix for 3 no less, no more.

I would use 3. You can add 2 tbsp of water if you are concerned about the moisture of the cake.

I bake a lot. I would use just 3 medium ones :)