According to Vekoma's website, even Lech Coaster can run three trains, but it has the same number of blocks as the standard SLCs and probably this too, which makes me think that this could also run 3 if a park wanted.
Looking at Vekoma's website, it certainly seems like an option to run Lech on 3 in theory. But when you look at the throughputs, in theory it works out as:
1 train: 500pph
2 trains: 1000pph
3 trains: 1200pph
So, theoretically (ie - running perfectly), a 3rd train only adds an extra 200pph, whereas a 2nd adds 500pph. Even then, running it perfectly would be hard to achieve (I'd imagine Lech could realistically get 850-900pph consistently). Because the extra block is at the end of the circuit, it also means that people would be waiting at the end on a block for longer, which usually bores people.
There's also the fact a 3rd train costs a lot of more, in terms of actually buying it, and then continuing to maintain it.
SLCs have clunkier restraints, more awkward trains and longer walks for staff to check restraints (they have to walk in between rows). So any benefit of having a 3rd train is basically lost by the fact your can't work quickly in dispatching trains on it. Blue Tornado is slightly different due to the extended layout, but again, the addition of a 3rd train is minimal.
For these STCs, even though the restraints are better and the trains are less awkward than an SLC, staff still have much longer walks to check restraints. I'd imagine again that any benefit of having a 3rd train is immediately lost in reality, unless you have a ton of staff.
So for this, and likely all STCs of similar length, I don't see them running 3 trains. Even if it theoretically could, there's no practical benefit to it actually doing so 3.