For example: set up "Get 3 for the price of 2" with bundle like screenshot below:
https://treedify.com/selly/bundles
In bundle offer setting, it needs 2 product groups:
Add/remove product groups to create big bundle
Set discount option "Various discounts":
Group 1: Minimum quantity is 2 with no discount:
Group 2: Minimum quantity is 1 with discount 100%:
