Don't tell them they're taking the wrong action, tell them they're taking the right action at the wrong time.
The key to getting more 3rd party support is to start 4 years ago, right after the last major election. The second best time to start is today, right after a major election. The WORST time to start supporting a third party candidate is right before a major election in a swing state with no party presence. Tell them to get to work. They won't get to be another major party, and they won't change the electoral system, but they can scare a major party into adopting one, maybe two of their policy positions. Sorry but that's just the best you can hope for if you're trying to change the system by voting.
The Starbucks 3rd space idea has been dead since 2015-2018. It was a conscious choice.