I know how to calculate 2 pulleys ratio but when 3 pulleys I have a little problem.

For example driver pulley is 10" diameter driven 30" pulley the ratio would be

3/1 or 3:1

How are about driver pulley is 10" turn the 20" driven pulley and the 20" pulley turn the 30" pulley.

How do you calculate this problem?

Thanks