You could make third parties viable with an alternative electoral system such as Ranked Choice voting.

Replacing First Past The Post voting with an alternative voting system will eliminate any chance of a spoiler effect. People could feel safe in the fact that if the party that best represents them doesn't make it, their vote will continue to be counted throughout the election.

More political parties means more representation, and a bigger portion of the population being involved in the electoral process.

More competition means higher quality representation, even from established parties.