WMC 2018 Senior Problem 17

A triangle number $n$ is the sum of the first $n$ positive integers. For example, $10$ is a triangle number because $1+2+3+4=10$ . A repdigit number is one where all its digits are the same. For example, $55$ is a repdigit number. There is just one $3$ -digit triangular repdigit number. Find the sum of the digits that make up this number.