Connecticut is holding a special State Senate election on February 26 in the Fifth District, which is centered on West Hartford. The Libertarian Party nominee is Jeff Przech, and he has petitioned to be on the ballot as a Libertarian, although his petition hasn’t been verified yet.
The Independent Party, which is ballot-qualified in that district, has also nominated Przech, so Przech will probably be on the ballot twice, and voters can choose to vote for him either as a Libertarian or as an Independent Party nominee. See this story. In 2018 practically all of the Independent Party nominees for federal and state office were also Republican nominees.
The new three-party system is coming to the USA Presidential Elections in 2020!
https://www.allpartysystem.com/one.php
I think that’s first for Connecticut, where another 3rd party endorses the Libertarian candidate. There was a state representative candidate back in 2010 who was endorsed by the Libertarian, Republican, Independent, and Connecticut For Lieberman parties, but he was a Republican that the LP and other minor parties were endorsing. I don’t know of another instance of a Connecticut LP candidate also appearing on the ballot on another party’s line.