There are at least five fairly obvious reasons why this footage should be treated as suspect:
(i) Look at the smoke/dust cloud behind the buildings. It runs all the way from the top of the WTC to the base. This only occurred AFTER the collapse of the first tower. Before then the smoke was confined only to the upper section of the buildings, as seen here:
(ii) Look at the top of the towers. Can you see that vertical protrustion between them, in the smoke? At first glance it looks like the antenna on the north tower. But it doesn't appear to be in the correct position (namely the centre of the roof). I believe that it's not the antenna but a clumsy dividing line - evidence that one of the towers in the footage has been doctored in afterwards.
(iii) If the two towers in the footage are supposed to be the north and south tower, how come the black patches on the building face appear to be at the same level, when the towers were clearly hit at different levels?
(iv) And while we're on that subject - look at the way the smoke billows from the top of the buildings. Doesn't it appear just a little too symmetrical, especially given that the towers were hit at different heights?
(v) Most damning of all - this site looks like a crackpot conspiracy exercise. Funny how it's always those folks who see these blatantly-obvious things that millions of others miss...
In short, I believe what you're seeing is a doctored version of the footage taken as the south tower collapsed (note the way the camera draws back) - which explains your "third" explosion - simply a dust-cloud thrown up from the debris at street-level.
Someone has added the other tower - possibly a mirror image - afterwards.
The prosecution rests, m'lord.