Steroids are contraindicated if there is any sort of bacterial infection.
I think that knowing where the injections were will go a long way into your current diagnostics. X-ray has various purposes for the neck–to identify trouble areas a little bit less expensively than ultrasound and to be used by a radiologist to measure looking for spinal cord compression. Ultrasound can see whether you have any effusion on a suspected trouble joint area. So, in your case, you might just want to go with ultrasound of the cranial and caudal joints at C3-4-5 unless your vet wants to start all over to build up a complete record.
It’s not that unusual that you’d do the injections separately. Say horse has a pretty obvious issue at C3-4 and not so bad at one of the others (I’m not sure if you mean he was injected at C2-3 C3-4 and C4-5 or C3-4 C4-5 and C5-6). The options are to do one-sided injections of the affected joint space, bilateral injections (which was done with yours), because often times that gives more relief even if the “problem” is on one side. Or to do all the clearly problematic and more mildly abnormal joints at once to just throw the book at the issue.
In my horse’s case, he had a clear OCD at C3-4 on the left side. There was effusion but no OCD at C4-5 and C5-6, also on the left side. My vet opted to 6 injections (bilateral joints for all 3 locations). Her reasoning was that she’d rather do that than do just C3-4 and then not be able to tell whether there was much improvement and then have to go back and do the others. Or, maybe she’d get lucky and all we needed to do was the one. By doing all 3, we will never know if we could have gotten by with fewer injections.
I had a bit of an insurance claim time crunch, had been drawing things out with alternate therapies and theories, and so we all agreed to just do the 6 injections and hope to be done with it.
In your horse’s case, it could have been a situation of trying it piece by piece and then discovering it took all 6 injections to make a difference. That suggests that you’d be looking to do 6 again, but you can see on ultrasound if there is still effusion at each of the previous sites and you and your vet can make the call. And if there is something like an OCD in there, you can also see that on ultrasound.
I don’t know why the vets each used a different product other than they had a specific preference for the type of steroid to use. I know several vets who prefer a certain steroid in the neck that my vet does not particularly like for that purpose, and she told me why she preferred a different one. If the horse’s records are spotty and horse is bouncing around between vets, that could explain the different agents used.