"I'd trust them about as far as I could throw their corporate HQ building to have any honour in employee dealings."
So I was a Three employee (non retail) Back during the failed O2 merger in 2015. I can tell you from personal experiance that you'd be 100% right in this.
As a result of the failed merger Three did a Firesale of outsourcing everything they could.
Customer service before that was literal award winning and full of passionate people who escaped the outsourcer were notoriously horrible, who did O2s customer service. Then 1 day senior management took them into a large meeting and excitedly told them the great news, they were being TUPE'd back to the same outsourcer. Somehow then CEO Dave Dyson was surprised someone launched a chair right at him over that news. This outsourcer was used cause Three knew they couldn't meet SLAs so meant the outsourcer had to pay fines to Three so even more money.
A while later, the day after they made a huge deal about World Mental Health Day, bringing in special guests and doing several workshops for their employees talking about the importance of mental health, they announced a large chunk of people were being made redundant by Xmas. I'm sure that helped with people's mental health.
I myself was made redundant a different time, and it was a long drawn out process, which made it so much worse. We lost people jumping ship who weren't replaced, so alot of us ended up off sick due to burnout with the extra work we had for months. There was an agreed severance packages Three would pay but after everything was signed they tried to renegade on some stuff as they took away weekend and oncall work. That one bit them on the arse as we went to ACAS who reviewed the deal and then not only said we were legally due that money and would easily win any legal fight with a literal baby being our solicitor, but despite it not being in the contract, we were legally entitled to more money due to bonuses being left out, and at the enhanced rate Three initally promised (4x the statutory severance package) so that backfired badly for Three.
I was literally told by someone I was being made redundant because "We can get 3 people in India for the same cost as your salary". I later found out they got 1/6th of my output from those 3 employees combined.
Oh and the way they did layoffs was sneaky AF. To avoid being in the new about layoffs Three didn't lay people off. They TUPE'd us all to outsourcers who would lay us off instead. The outsourcers tried to make us make statements to Three saying how great life was with one saying "It's so great cause they gave us beds in the office so we can work longer". When they tried it with my team everyone bar me told them to GTF, apart from me. I already told them GTF several times already so somehow they grew a braincell and knew to avoid asking me.
It was a horrifying and mentally damaging experiance and I fear for the Three staff who aren't senior management cause duck them. Infact during consultation the senior manager I was working under (who is still at Three) said he felt our stress cause he didn't know if he could keep his huge home where he hosted office wide parties, or if he could keep sending his kids to private school, whilst some of us were struggling to pay mortgages for tiny homes. So I hope the senior management team gets screwed over badly, but I do sympathise with everyone else, I particular retail staff who will be hit the hardest.