I've been looking into these over the last few days, and it looks like they are in some way related to Project Wingman, who store their vehicles in the same compound when not elsewhere in the country (one recently spent some time in Scotland). The airport website also carries an aerial photo of the site, with a vehicle in what is known to be Wingman livery surrounded by these three.
There's a press release from First Group in February stating they are handing over a 2006 Volvo to the charity as their third vehicle, which seems to match the livery of this one (although as usual for press releases, the registration is obscured). No specific knowledge on the Transdev ones, but it seems likely they may be additional vehicles or, given the similarity of age to the native examples, parts donors.
The identified one is ex Transdev 3601, whilst I think I've narrowed down the other one to one of three vehicles, ex 3604-6 inclusive. These were the original Harrogate 36 vehicles, refurbished and refronted for CityZap, then RedExpress. Four, 3601 and 3404-6 were then temporarily used on Dalesway 66, hence the yellow strip on the upper deck windscreen. It appears the driver training decals are leftovers from a final use at Transdev. All subject to me having read captions correctly (and those captions being correct!), of course, but that's as narrow as I can get it.
Chris