Which expression can be used to model the phrase “ the sum of three and a number”?

x + 3

Step-by-step explanation:

A 'sum' means addition, 'a number' is represented by a variable, in this case, I chose 'x'.  I could have picked any variable I wanted.

The sum of three and a number means:  three plus some number, which is

 3 + x       in math language.     We write variables before constants, so it gets

                  rewritten as x + 3