Yes, Dorothee is fully right. Trios are nice as choice of partners is concerned, the recommendation by Bernd is justified.
But in respect to the growing-up of the offspring in presence of the old fish it may be not the best choice. It depends: sometimes, especially in bigger tanks with many hining places, they manage to survive, often it’s only a very few.
For this reason we (and Bernd too) do not follow this method at any rate; we take the old fish out if we want to raise young efficiently. It’s always a risk not to do this. But we don’t do that in other cases: I had pairs (not trios) of nagyi or paludicola in a rather plant-crowded small ten liter tank, that produced more than fifty young; I could harvest them some months later. And I had a similar ten liter tank with 2 pairs of freshly cought P. gunawani, that contained some weeks later not 4 but 12 fish.
As I say, it depends on the structure of the tank, but in the first line on the different habits of the old fish. Some leave their offspring unmolested, others not. With a pair of P. sumatranus I watched the female after free-swimming of the young intensively search fo each of them to eat it.