It starts at the local level. National 3rd party candidates are just spoilers used to split the vote enough for one side to win when they wouldn’t in a 1v1. Until the third party builds enough momentum in city->county->state elections they will never be viable nationwide.
Instead of focusing on the national, they need to pick a few places to focus on and grow viable candidates throughout the whole process. If they can flip one state to the third party, then we might start talking about their viability.