In this scenario, the store owner offers a discount $10 for the product "MIrror" if customer buys quantities 3 from the product Clock.
Step-1 : General tab
Since the product gets discount based on the purchase of another product, the discount method you will have to choose should be Dependent product based discount.
Step-2 : Condition tab
Set the discount conditions like
Buy = Any - Equal - 3
Product(s) from = Clock
Get discount in = Mirror
Step-3 : Discount tab
Set the discount value $10 for the product Mirror.
Step-4 : Frontend demo