1st of all the way you summarised the days of the split are bs, second you wont grow overnight like a balloon and get “slower” or have “less endurance” or anything like that. I can almost assure you, you already have bad/average endurance or stamina or anything else because as you said you don't even train anything. If you want to achieve something then you should train that thing specifically such as lets say stamina-you do activities that challenge it such as running or activity that pushes you beyond what you are already comfortably capable of doing (gym also does that); lets say strength - you lift weights with the intention of hypertrophy, it goes like that for everything and no its not like a lightswitch “you train one thing but for the cost of the other” it doesn't work like that. If you have any questions i can explain them more deeply and in terms of split the only split you can basically do for 3days is full body which i can make for you