First week with our rescue pittie - it's a lot but I think we made the right call

We brought home Duke from the shelter exactly 7 days ago. He's 3 years old, listed as a "pittie mix," and honestly he's a lot more dog than i expected. First two days he just hid under the bed and wouldnt come out except to eat.

Day 3 he finally started exploring. Day 5 he jumped on the couch next to me and just stayed there for 3 hours. That felt like a win. He's still pretty skittish around sudden noises and he had one accident in the house but overall he seems to be settling in.

We have a meet and greet with a trainer next week to work on some leash stuff because he pulls pretty bad. Anyone else adopt an adult pittie? How long did the full adjustment take for you? I read something about the "3-3-3 rule" but i don't know if that's actually a thing or just something people say.