1. Start with 

(3+x)+y

Commute (interchange) the 3 and the x and get

(x+3)+y

------------------------------------

2. Start with 

(3+x)+y

Commute (interchange) the whole (3+x) and the y and get

y+(3+x)

------------------------------------

3. Start with 

(3+x)+y

Associate the x and the y instead of the 3 and the x and get:

3+(x+y)

------------------------------------

Using the commutative principle means to interchange two numbers.

Using the associative principle means to move the parentheses from
around one pair of numbers to around the other pair of numbers.

Edwin