Letting batter with baking soda wait before adding into the oven
I doubt the raw batter will survive 35-70 minutes out of the oven.
Many times when I made homemade cake, things like

forgetting to preheat the oven

taking too long to scrape all the batter into the baking pan

taking too long decorating the top of the cake batter when its in the pan


rewards me with a flat cake.
On the other hand, after adding the wet ingredients to store bought cake mix, surprisingly, letting the batter sit for a long time (like 30 minutes) before popping it into the oven, doesn't really change the finished product!
What I recommend you do is have your wet ingredients combined and dry ingredients combined all ready in two separate containers. For each cake, mix a third of the wet ingredients with the dry ingredients; this would be more practical then making the batter three times.