The initials 'LNB' stand for 'Low Noise Block' which is the downconverter module that the satellite signal is aimed at by the dish and which converts the signal so that it can be fed via a cable to the receiver.

Depends on what your roof is made of, but may work. Also, any build up of water on roof, if there is anywhere in line of site it can accumulate, will severely degrade signal.

Yes, you do (assuming your installation only has one cable to start with). You need two co-axial cables to carry the two channels at the same time.
Fitting a second cable yourself is fairly easy, as long as the LNB fitting on your dish has two (or more) outputs - many do. Just be careful not to disturb the alingment of the dish, or you will loose the signal.
If needed, you can replace the LNB, but I guess this increases the chance of the disk needing to be realigned.

The answer that says you can do this with a splitter is nonsense - unless you want all three TVs to be showing the same channel!
The picture that goes with the "Freesat Installation Kit" on the Argos website shows an LNB with a single output. If you want to run three (or four) TVs you would need to replace this with a quad output LNB. You would then have to run a different cable from the dish to each TV.
Also, you will need a Freesat receiver for each TV (unless a TV has a built-in receiver).
Note that if you want to use a Freesat recorder, it needs cables from two of the outputs (so you can record two channels at the same time). If you have two recorders, this would use all four outputs from the LNB.

Some of the most expensive TVs have both Freesat and Freeview built in. If yours is one of these, then you won't need a Freesat box (just a dish). However such TVs are rare, so you probably will need a freesat box as well.

No, it’s not really compatible, because the picture shows that the "Freesat Installation Kit" has an LNB with a single output. That would only allow you to view (or record) one channel.
Obviously you want to be able to record two channels at the same time (or watch one channel while recording another). To do this you require a dish fitted with a dual output LNB or quad output LNB, with two cables running to your recorder.
You could easily replace the LNB that comes with this kit with a better one or you could buy a set-up that comes with one to start with.