We can begin by assigning a variable to the unknown number; in this case a number. Let's call this unknown number x (as that is the variable that the answer choices are given in terms of). Once we have our variable, we can begin to translate the written problem into numbers.
"Three times a number" implies multiplication of x by 3.
"is" indicates an = sign.
"three more" implies addition of 3.
"twice the number" can be translated to multiply x by 2.
Adding all of these together we get 3x=3 + 2x.