yeah, good luck!
If it's got a plug-and-play socket in the loco then it is possible to take the DC blanking plug, cut the traces and solder on your wiring to your 583S decoder - but that of course means you can't use the blanking plug to go back to DC. personally I'd make up a new plug using some header pins and a small piece of copper-clad stripboard. At least it's a straight 1-to-1 link of each wire from the decoder to the plug. ** Oh - beware that I think there's a note somewhere about one of the pins should NOT be connected on the RS-3. If I remember correctly recent DG583AR decoders come with one pin cut off to avoid the problem.
If it's an older loco without plug-and-play then you're probably looking at re-wiring everything. Not difficult - the first loco I ever did was an old Aristo U25-B that needed hard-wiring.
Re the ebay advert, I think the problem is that the seller mentioned "3x" in the title - generally people will take this to mean that is what's included in ONE auction lot, and if there are multiple quantities it is a multiple of the complete lot. Bit silly to then mention "3 available" in the text, which of course isn't true once one is sold! Like I said, I had to read it twice to convince myself. Shame - I'd have snapped up a bunch of 3 decoders for £45 for my stores!