Single Day pass or not?

I’ll be staying in Tokyo -> Osaka -> Kyoto -> Tokyo over 3 weeks. I won’t be leaving Tokyo until 4-5 days into the trip. Then saying at each place for a little less than a week.

My question is, since I won’t be using the train daily, would it be more cost efficient if I buy day passes for the day of travel?

I feel like buying the 21 day pass is a bit much when I’m only going to use the train maybe 10 days max out of the 21.

Also, if I buy a pass, if I don’t use it until 2-3 days into my travel will it activate then? Or does it start the day I select even if I’m not using the train?