It's not the DNC and RNC that make third parties not viable, it's the "first past the post" voting system--one election and whomever gets the most votes wins. That forces people who might consider a third (or fourth or fifth) party to choose between "give an extra vote to a party you like most in their bid to reach 1% of the vote" or "give an extra vote to one of the parties that might actually win, allowing you to have some voice in who's in charge."
If, for example, we used a run-off vote, that choice would be eliminated. In the first vote, you could vote for anyone your heart desires. If you really like the Constitutional Pirate Party, vote for them and get excited by their attempt to pick up a percentage point or two. Then, in the run-off between the top two vote-getters, you can have a voice in who actually runs things. That would allow parties aside from the Democrats and Republicans to slowly build coalitions and popularity and maybe one day challenge to be among those top two vote-getters.
Beyond that, a parliamentary system also tends to be more friendly to smaller parties, but we aren't completely restructuring our form of government any time soon, so it would be better to advocate for something like run-off voting or ranked-choice voting. Some states have begun experimenting with those types of systems--maybe one day there will be enough support for it at the federal level.