I don't think they actually were called the monster trio. They were, on separate occasions praised for their superhuman abilities. That's really about it. I think it's just a moniker the fans came up with.
I think Namy called them like that once,when Luffy tried to help them somehow and she got hurt and told him don't compare me with your monster trio.And then I guess the fans liked it and sticked to it.
It was the crew that called them "The Monster-Trio", and it was stated in the One Piece wiki, as you can see here:
All of them have at least one talent at which they are virtually perfect at and each member is, more or less, a formidable if not downright fearsome fighter in their own right. While all of them have amazing strength in one way or another, three of them, the captain, the swordsman, and the chef, have such astonishingly inhuman strength that the rest of the crew have dubbed them the "Monster Trio".