@Trevor350
Use rulesets.
Ruleset for a friday sets min. stay to 2 nights, max. stay to 3 nights. Apply a discount for a 3 nights stay.
Ruleset for saturday limits to 2 night stay (or if already default limit in the listing: no action needed)
If you ONLY rent weekends, then limit check-in days to friday and saturday and check-out days to sundays and mondays. Set max. stay to 3 nights and apply discount for 3 nights stay. Then you can apply the ruleset to a long time period (instead of applying to each weekend)