I want to bake a cake with 3 layers, but I only have one baking pan and thus I will have to bake them one by one (already thought about making one big cake and dividing it into 3 parts but I pref not to use that method). The problem is it takes 35 minutes to bake one layer and the batter contains baking soda and baking powder and I'm worried it will affect the outcome since I'm not baking the other 2 layers immediately after adding the baking soda. So my question is, can I let the batter for the other 2 layers wait before adding to the oven? Making the batter and separating into 3 and then adding the baking soda and powder isn't an option since I would have to mix it again which will affect the result. And I also don't want to make the batter for each layer separately. 
The last layer will likely be pretty flat, especially if you're using a recipe with whisked eggs for some structure. Any air bubbles and structure you whisk into the eggs will be gone by the time it's been standing on the counter while the first two layers bake. And you will lose some lift from the baking soda reacting with acid in the batter. You will get cake, but it will not be as good as it could have been. I would personally bake it in a taller pan and slice after baking. That will give a much better result than letting the batter stand for over an hour before baking the last layer. 