There are a couple of threads on this already. Something I've never tried, but one suggested solution was to solder a wire internally between the 1st two ports on a 4 way diseqc switch. I think there may even be an adapter to do it externally.
An easier option in my opinion is to ditch the monoblock and use three separate LNBs, it saves the faffing around.
Oh, a little search of the forum brings up this thread:-
Monoblock on 4:1 diseqc switch?
A good solution I think. A switch made especially for your purpose.