The way three or more factors are grouped when multiplied does not affect the value of the product. For example, consider the product $2\cdot 4\cdot 6.$ Grouping $2\cdot 4$ and multiplying it by $6$ produces the same result as grouping $4\cdot 6$ and multiplying it by $2.$ Since the Associative Property of Multiplication is an axiom, it does not need a proof.