How do I introduce a new dog into the house?

We currently have 2 dogs that are 5 years old. They are well behaved but are dog-reactive. Anxiety, not aggression.

An elderly family member is moving into assisted living and cannot keep his dog, so we have agreed to take her. There's no other option and she will be put down if we don't. I've met this dog twice, she is shy but sweet. Also around 5 years old. Unfortunately she has never really had any training and has been mostly kept in a single room or outside.

I'm comfortable with introducing them, dealing with anxiety during the day, etc.

My concerns. My dogs have kennels but we stopped closing them at night/when we're gone 3 years ago. I'm struggling a little bit at the best way to handle nighttime when we can't supervise. We will work on kennel training the new dog, but I'm not thrilled about the idea of our dogs roaming free while new dog is in a kennel. I think that could cause some anxiety for new dog. Maybe we can put her kennel in a shut room so that the other dogs can't access? We have never allowed our dogs in the bedrooms upstairs and although bringing the new kennel into our bedroom during this beginning time seems logical, I'm not sure I want to start that habit when the others aren't allowed.

New dog also has some bad habits that we are going to have to work on, including scratching up the walls and housetraining. I'm hoping a lot of it is from boredom so fingers crossed there. When our dogs were puppies we kept them leashed and with us if they weren't in their crates and they learned pretty quickly. Is this the way to handle it with an older dog as well? Will the new dog learn from our dogs or will ours pick up bad habits?

Appreciate any help or tips!!