If I play a Flamy Fury and use a weapon indicating Blast, will every hit monster get three burn tokens or only one?

My group decided you only burn one monster, since the card says something like "Play after you attack a monster, place three burn tokens on that monster " rather than "...the attack gains Burn."

Of course, they still insist on not giving the Burn ability ranks, so I guess there was no simple way for them to do the latter...

Putting 3 burn tokens on every monster you hit probably isn't overpowered, but as far as I can recall, all the weapons that can inflict Burn are written as if it were...