Answer:
First. Let us prove that the sum of three consecutive integers is divisible by 3.
Three consecutive integers can be written as k, k+1, k+2. Then, if we denote their sum as n:
n = k+(k+1)+(k+2) = 3k+3 = 3(k+1).
So, n can be written as 3 times another integer, thus n is divisible by 3.
Second. Let us prove that any number divisible by 3 can be written as the sum of three consecutive integers.
Assume that n is divisible by 3. The above proof suggest that we write it as
n=3(k+1)=3k+3=k + k + k +1+2 = k + (k+1) + (k+2).
As k, k+1, k+2 are three consecutive integers, we have completed our goal.
Explanation: