The history of train packs is littered with three or four car packs, because most 'trainsets' can't realistically handle any more than that.
Hornby HST - was 1 coach and 2 PCs, then 2 coaches and 2 PCs
Hornby Pendilino - four car
Hornby Eurostar - four car
All three had extra coaches available seperately and the 'Javelin' is no exception to that.