If your player just rolled a single d8, added the +5, and tripled it - he did it wrong. You actually HAVE to roll the damage three times. If you don't - you don't get the right damage curve.
Perhaps i have missed an errata somewhere but doesnt "x3" mean "multiply by 3"? Maybe im just crazy but...
Hey man just get out your DMG (you do own one right?) and look up the Tomes and Manuals in the Wonderous Items section in the back of the book.
Why am I the first to suggest this seemingly obvious method?