To be third-wheeled is to be intentionally left out of activities as part of a friend group. If you're third-wheeled, you're most definitely the
convenience friend. They'll make you desperate to hang out, but never actually include you in anything. You can expect to be asked to hang out only if nobody else wants to.
If you notice you're being third-wheeled, it's probably a
toxic friendship and you should get
out of there.