A triple bracket expression has three factors, such as $(x+1)(x+2)(x+3)$. The safest method is to turn it into a double-bracket question first.

Do not try to multiply all three brackets at once. Expand two brackets, simplify to a quadratic, then multiply that quadratic by the remaining bracket.

When three linear brackets are expanded, the final answer usually starts with an $x^3$ term. A quick check at the end is to write your answer in descending powers: $x^3$, then $x^2$, then $x$, then the constant.