I mean if they don't wish to sell to a "big group" surly it makes better sense to combine all of them into one group "Red Bus Company" maybe, it surly would allow better fleet use if one needed a bus and one didn't it could move without looking weird and with them all working in a similar area, the finances would be better for them all?
Hard to say, really - presume it works for them as it stands.
There may be some advantages in company law / tax to being three small companies rather than one big one (I'm not expert enough to know) and presume they can informally take advantages of working together for bulk buying and that sort of thing, and they already share premises so presume some workshop equipment and the like is shared. I don't know their operations well enough to know if they have a few 'generic' buses that can be loaned from one to another if needed.
I don't know how easy it would be to run as a co-operative model, as in one outward identity but multiple legal operators - I'm aware there were a few like this in the more distant past, A1 in Ayrshire for example. Although I think that before the 1985 Transport Act, the (equivalent of) a service registration could be held by multiple operators, with them taking turns to run different journeys on different days, and I'm not sure that would be possible now.
And with no detailed knowledge of their operations, so no reflection on them at all, and I've never noticed them featuring in the 'legal and licensing' bit of the trade press, but there can be less risk of enforcement action if you have multiple small operations on different o-licences. I can think of one operator who may or may not still be in business who seemed to be quite a regular at the traffic commissioner's a few years ago, and they had a few o-licences revoked, but there were enough other o-licences for (legally) separate businesses within the family to mean it didn't really seem to affect them.