[targonrotts]

there are a few registries out there other than the 3 you mentioned.
these are the most common 3 we hear from the most(at least in my neck of the woods).

main differences i would say is the testing methods and requirements.
TDInc requires a test and 3 observed visits. TDInternational is like a more advanced CGC. Delta does have a more rigorous testing, i hear.

you need only be registered with one for the insurance coverage. i belong to TDInc and in our rules if you are under our name during the visit, you are only to wear our ID tag for insurance coverage reasons.

i am glad you are looking into therapy dogs. it is a great thing to do.
we do once a week mostly...except during show season it becomes every other week.
we do hospitals and occassionally schools or special events/nursing homes.