There’s plenty else they could be doing… outreach in off-years, for example. Start on campuses building awareness and building the kind of word-of-mouth and grassroots supporters you really need for a campaign. Having your name on the ballot isn’t enough. Having rallies isn’t enough. You can’t ask the people to come to YOU, and the media certainly won’t give you any coverage… you have to reach out to THEM.
That is true… of a traditional campaign. But we live in an era where people can get millions of devoted followers by twerking on a webcam. A savvy third party that uses the internet effectively to build followers and then spreads into the greater population through word of mouth could conceivably work. Heck, it’s not all that different from how Trump managed to build his base.
I’m not sure exactly what such a thing would look like for a third party candidate with some kind of scruples, but it shouldn’t be IMPOSSIBLE.