Hi there. Immigration @ CGK would be similar to that in DPS. I believe you'd go through immigration, pick up your luggage, go through a transit lane, and re-check your luggage at the domestic terminal. 3 Hrs should be more than sufficient to go get your on-arrival visa (~20 mins), immigration (~20 mins), and luggage/custom (~30 mins).

At CGK, international and domestic terminals are not within walking distance (i.e. gotta ride the airport bus). HOWEVER, for some reason domestic GA (Garuda) also occupies the international terminal, so you should be OK.

As far as GA itself, I'd say it's probably the best among the mediocres - i.e. among the other Indonesian carrier choices you have, they're better in terms of punctuality and equipment age/condition. You should have an acceptable experience.

Enjoy your experience, and bring plenty of shorts & t-shirts!