Three passive systems should be able to triangulate threats with enough precision to launch ARH missiles at...
Most low flying missiles require low powered radar to avoid flying into terrain, plus many cruise missile types use radar to navigate to target... having three sensors the emissions could be triangulated to locate targets fairly accurately... with proper spacing it should be rather effective even with no direct line of sight to the target.
Note the SA-13 and SA-9 had all sorts of sensor boxes on them to give early warning of incoming threats based on radar and radio and even datalink emissions.... you don't need to hack the datalink to just detect the signal and identify its source...
No, we are talking about two completely different things.
What you are describing here is a is an ESM system i.e. something that sense ENEMY own emissions and extract data from it.
Bystatic/multistatic radar are another thing: they are basically made by just the receiver part of a normal radar and receive their data from an outside source i.e. from a radio wave emitter.
In case of S-350 we are talking about a multistatic non-cooperative one i.e. the most advanced model that receive the signals incoming from different radio waves sources intended for normal, civilian use, examine and map them and detect incoming planes not by a direct return eco but as an interference in such a landscape.
It have a great efficacy, above all against stealth planes but also some limitations or better inherent peculiarity of its own that have to be considered in advance to get the result.
Good news is that the huge development work needed to make it operate as intended is already been done and now is finally reaping time.
So they are only using the middle missile?
I suppose if the focus is cruise missile defence then the middle and smallest missiles make the most sense...
I thing is just what is written in a single source, probably they describe just the standard model used for the intended main role, not the whole array of weapons it can or will use.