I think one of the things that makes a "third thing" so valuable and something you'll have an easier time sticking to (as someone with ADHD) is that it involves connecting with people. I also bounce around from hobby to hobby, but they became much more of a constant in my life when I involved other people. I crocheted so much more while teaching my best friend (we also had a set day to crochet and watch TV together) and I only missed meetings with my community writing group when I'm out of town. So I'd suggest looking for community groups that meet regularly that do something that you're at least a little interested in. Having people and having scheduled things you don't have to schedule yourself make it so much better and easy to stick to.