do you mean 3 hour stretches per appointment, adding up to potentially 9 hours per day? If so, then I'd think long and hard about it. 9 hours is an awfully long time for a mutt. If you just mean you'll be gone 3 hours a day or so, then I don't really see any problem with that. You don't have to be a total prisoner in your own home just because of a dog
Get him a buddy and he'll cope much better with your absences.