Let me firstly frame my response by saying that its something of a gamble as all airlines make arbitrary changes to their programs with little notice so what holds true today may not tomorrow.

Secondly I think the trend is for airlines to devalue their frequent flyer benefits as bean counters take control and the original purpose of encouraging loyalty is "forgotten".

I balance flights to maintain top tier status in two different programs, one is SQ. I used to support just one (NZ) but because of the two points above formed the view this was too risky. That also gives the flexibility to switch as necessary. I would also add that it seems availability of award seats is better if you support the carrier directly, eg SQ flights are more available if you have SQ points than other *A carriers.

To maintain PPS on SQ you would probably only need 3 trips in business class per year. So I would do that first then the balance on either LH or VS depending on your preference. SQ does hold about 50% of VS equity so there are some VS lounge access benefits from SQ but it seems the relationship is becoming a little fractious so it may not last. I added LH because many people seem to think the points accumulation is better there. You could under SQ's program put points there and still get PPS status but the issue there would be award flight availability to consider.