Start by having Mr. ESC proceed to picking up the fire extinguisher, he won't be the one to use it but it helps to have it on hand for now. After climbing the wooden box (which should not be pushed yet) and up again further to rescue the heavy man, have him push off the stacked boxes to the left so they fall to the ground on top of the box below. Then send both the heavy man and Mr. Esc to where the child is and rescue him.
Now with the rudimentary stuff out of the way...
Order the heavy man to push the stack of boxes right until the lowest wooden box of the stack is adjacent to the fire (therefore, half of the steel box is hovering right above the fire). Once in place, have Mr. ESC climb the stack so that he may push the topmost box over to the right so that it lands right on the collapsible portion of the floor (do not move Mr. ESC on there). Next send the child up onto the stack via assistance of the heavy man, then make sure to pass on the extinguisher to the child. Now have the child stand on top of the recently pushed box, he'll be safe due to his light weight, then him put out the adjacent fire.
With the fire put out, send the child all the way to the right where he is to collect the rope item and hit the nearby switch. After the activated switch extinguishes the flames on the lower floor, make sure to move Mr. ESC off the stacked boxes (remembering to jump if going right) so we may have the heavy man push the stack over one more space, thereby setting off a a convenient reaction which should drop a wooden box to the floor below while covering up the newly created gap.
As for that last spare wooden box, have it pushed right until it falls to the floor below. Have Mr. ESC dangle down and land onto the box (even if you've already set up the rope) and have him push it left until it covers the rightmost, since pushing it any further would electrocute him, of course. With box out of the way, you can now send the already two rescued victims down below via the rope and to the exit as they are no longer needed. With a gap of two danger spaces standing between Mr. ESC and the final victim, have Mr. ESC climb onto the recently pushed box then leap over to the left allowing the final victim to be rescued. Carefully order them both to leap back over to the right, then escape via the exit. Good luck!
Fry: "I know that monkey, his name is Donkey." Farnsworth: "Monkeys aren't donkeys! Quit messing with my head!"