You will have an extra mino when starting your 3rd PC. It's because two 4-line perfect clears require 20 minos, equivalent to 2 bags and 6 minos.
These are some patterns that you can use for your 3rd PC. The credit to 5+ piece patterns shown here goes to JimothyJimothy. Refer to this page for some context. We have also selected some notable 4 piece setups from Best Chance Field.
Makes the same shape as the O pattern.
thanks to jstrisgg for pointing out that JSTZ alone is 100%