Just to give an engineers view, I did a fair amount of study on this during uni. The fire precautions in the tower was ok the intumescent cladding was a bit shoddy but in 99% of fires it would have been ok. With the damage caused by the planes strike the poorly fitted cladding came away.
Modern Fire protection mainly looks at single source fires, steps to control it and give time for the people above the fire to escape, they are not based about 5 floors starting in a fully engaged fire.
The largest limiting factor in a building fire is the amount of air if can get not the amount of fuel that can burn, the aviation fuel was a great accelerant to get the real fuel load burning the paper and the flame retardant office furniture.
With 5 floors worth of windows blown out there was plenty of air getting in and a fire burning paper and wood at stoichiometric rates you an see temp of over 1000C in the flame, steels structural fail tep is well exceeded so is no shock that the floors involved with the fire would collapse.
The structure of most high buildings is made to cover the static weight of the floors above with a extra margin for safety, around 50% if i remember right (I'm a fire engineer not civ so I could be out on that) but that's static weight the force of the top dropping though the 5 damage stories would have been like a bomb.
So why did the 3 tower drop? The same as the first 2 massive uncontrolled fire, and plenty of big holes for air to get in.
There is no need for explosives.