They typically had to submit signatures to get on the Primary ballot (or, I assume the caucus), although not as many. Here in PA the Ds and Rs submit about 2,000 signatures to get on the Primary ballot, 3rd parties have to submit about 25,000 or more to get on the general election ballot.

“In case of emergency, break glass”. Just hope the GOP doesn't wait until late October because by then the ballots will already be printed and the case will be closed.

I thought the Libertarian and Greens generally had access based on having had access and gotten votes in last election. I'm surprised to see that's not true.

I don't have interest in voting for a third party. But I recognize that historically they've served as incubators of ideas that were eventually adapted by one the big parties. A good example of this is Social Security. A bad one is Prohibition.

I also think that parties that can establish themselves in some greater than nominal way ought to have guaranteed ballot access. Though I don't necessarily think they should have automatic access to things like the Presidential debates or public funding.

Naturally enough, there's also a blog on this subject. I don't know enough about that blog to judge its quality, but it's out there. There are probably others.