Syntax definition cmpt3

Description: Syntax for maps-to notation for functions with three arguments.

	Ref	Expression
Assertion	cmpt3	class ( 𝑥 ∈ 𝐴 , 𝑦 ∈ 𝐵 , 𝑧 ∈ 𝐶 ↦ 𝐷 )