There is a correct set to use on these rails.
The genuine Vauxhall one's are designed to locate in the holes then lock in place.
They do come up on E-bay quite often( bought some myself 2 weeks ago) so keep an eye on there.
I don't think it matters which ones you use. It all depends on the size of the box/rack you are fitting.
If your putting a ladder on the bars for example, I imagine you could use 1 & 3 but for a small roof box you would use 2 & 3 or 1 & 2. There is no recommended combination in the book just a max weight of 75Kgs including the bars.