Is it ok to leave Christmas cake batter to sit for 3 hours?
I‘m not sure how much leavening the baking powder will really accomplish in this recipe but anyhow, that’s your „critical ingredient“. 

To be on the safe side, I would recommend you prepare your recipe until „day 2“, then on day three split the cherry mix and the remaining ingredients in two and do the final mixing in two batches so that you can bake them right away.

If you choose to finish all batter at the same time, just let it sit at room temperature, or, if you want to be on the super-safe side (eggs, > two hours) you can put it in the fridge, but that’s going to increase the baking time and aggravate the „burnt-edge-raw-center“ issue.

Now some might argue that for food safety reasons the raw eggs should not sit at room temperature for more than two hours, but as you are going to bake the batter for a quite long time, anything that might have grown will be pasteurized and the time frame is too short for the production of toxins.