The thing is complicated but not impossible.
I also have 3 LNBs and 2 are SCR, but I had to use, for ine of them (not my favourite, of course) the 2 Universal outputs instead of the SCR one.
The fact is:
1) Is surely impossible to manage more than one SCR by a normal DiSeqC switch.
2) The ideal would be to have two tuners in the VU+, one for the SCR LNB (where you can use FBC virtual tuners, in case), and the other one only for Universal LNBs.
In my case, this would be a pity, as I'd loose the 2nd outputs I have in the other two LNBs (let's assume the are both Universal because, as I said, I use the universal outputs from one of the SCRs). I'd need 3 SAT tuners! But I can't put 3 sat tuners on a Solo4K. So....
3) The SCR LNB is manageable adding the DiSeqC command to the tuner configuration (also for FBC ones).
The other ones, configured as Universal, in theory can be managed by the other 2 DiSeqC commands for their ports, but...
... in the past I'm sure everything worked well.
In the last images, I need to manually set the priority because the AUTO priority causes problems: if left in AUTO, if a recording starts by one Universal LNB and you zap to a channel which uses the SCR, the recording is stopped.
Similar issues if you start a recording by SCR and then you zap to a Universal-managed channel.
So, that said, as you see, the problem is only in case of a recording is needed from one type of LNB at the same time you need to zap to a channel managed by the other type.
To (partially) solve this problem, I used manual values in the priority field. But it's not perfect, even for recordings...
I assigned to 4 SCR channels priority 1, 2, 3 and 4 and I assigned to the other two Universal LNBs, priority 5 and 6.
So, as you see, the thing is surely possible for zapping (but remember: only one SCR works) but the recording/watching at the same time from different LNB types needs to be tested (from the same type there's no problem).
