The three companies have separate seniority lists. Group date of joining only determines redundancy (and staff travel!) within the individual company you work in- ie, baby pilots (max seniority 8 years) and regional pilots (all but the top15 or so, seniority less than 9 years) would not be displaced to make jobs for mainline pilots (average seniority about 15 years) if all the redundancies happened within mainline.
All this, of course, is true if LIFO is applied as the main criterion for redundancy; this is still subject to revision to comply with age discrimination legislation.