I think localised infections would normally appear within 24-48 hours after a procedure - I suppose 3 days is not beyond the realms of possibility, but maybe unlikely. I think too that when we ask for procedures to be carried out, we do accept a certain degree of risk with them - infections can occur after the most sterile of procedures. No one can gove 100% guarantees all of the time and lets face it clearing out the anal glands is,by its very nature, not a "sterile" procedure.
I do not think there is anything to be gained by accusing the grooming parlour, especially if everything was generally satisfactory. I suppose you could always mention it in passing on the next visit, but it is probably just one of those things.
I suppose you now know what is wrong, and that must be a huge weight off your mind.