For example: set up "Buy A get B at $10, applied to 3 B at max" 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
Discount option is "Various discount",
Group 1 is collection A, group 2 is collection B with discount & max quantity 3
Enable quantity box: