Oh...For potatoes..I like to nuke them if I need them quickly..or I simply boil them with the skin on (cleaned) and mashed them up. Or..I'll fry them in olive oil or coconut oil depending. To be honest, frying isn't that bad if you use coconut oil especially..a little bit goes a long way in a non stick pan..I swear a half tablespoon will fry up two big potatoes sliced up.
For plantains..I like them real GREEN..almost pure starch..little to no sweetness. I slice off the ends..and then slice the rest up with the skin on. Boil water with a dash of salt in it...then add the plantains..let them boil for like 10-15 minutes. You just slit the skins and they peel right off. Put salt and pepper on that, maybe some butter if you want.
Like others have said..if you are looking to make potatoes in bulk, then the best way is to foil them up and put them in the oven and bake a bunch at a time. You can then eat them cold or reheat them in the microwave.
Rice is pretty self explanatory...But, I don't typically cook rice in bulk..more what I'm going to eat at that time. I feel I can more accurately measure the macros of rice when it's dry. You can go by 1 cup of cooked rice as a serving, but sometimes depending on how much water was absorbed..this isn't as accurate as a dry measure or weight measure. Not sure if you are that spot on about shit though.