I have 3 of them too. There’s one who is REALLY aggressive at trying to get out. I found it recedes a bit with age. Our oldest cat when she was just a kitten, we kind of broke her of the habit accidentally. We got about a foot of snow one day when she was still very kittenish, and we opened up our deck door. The cat launched herself outside, and landed, all 4 paws in a snowbank. She got this extremely surprised look on her face, her back arched like she’d been confronted with an enemy and she just froze there for about 10 seconds, the literally leapt back into the house. After that, she is actually somewhat reluctant to go outside.
We have another one though who is about 4 now, we got her when she was about 1, and every time we try to open a door, she tries to get out, but she’s skittish enough that we can scare her back. But there have been three occasions when she got outside at night, we didn’t know it, and the next day she was standing on the front step beating on the door wanting to come in. But you can let her in, she’ll grab a bite to eat, and an hour later she’ll try to get out again.
I have one solution to the surprise knocks that works for me….if I don’t know that someone’s coming over, I don’t answer my door! Usually it’s just someone asking for money anyway, and if it’s a neighbor, they’re persistent enough that eventually I do answer it.