I have the app on iOS, macOS, and windows... Is there any way to sync my custom dice bags and dice between the three?

Thanks!

I don't know of any automated way to do it and I haven't implemented that myself, sorry. You'll need to do it manually by copying the files betweein the dice folders:

https://sophiehoulden.com/dice/documentation/customising_dice.html#custom_dice_f...