Turn bmx bike to 3 gear I have a 29” S.E  Big Ripper And I want to add 2-3 gears on it to ride hills . I don’t want to buy another bike but would like some ease in my older age .
You can buy basic 3 speed freewheels on eBay. With that and a claw-style derailleur you should be able to set up a 3 speed system. You might want to just get a different bike instead though.