I think a third party would be useful now, if it is used strategically, even though our system favors a duopoly. For one thing, there has to be a threat of withdrawing votes for the centrists to take us seriously. For another, the way things are heading, the Republican party, already a minority party, may be receding into marginal status, with the Democrats moving right to attract “centrist” Republicans. That would mean there is room for a new party on the left, which could draw progressives away from the Dems.
I think, though, it would be wise for such a party to start out like the now defunct Liberal Party in New York, which sometimes ran its own candidates and sometimes embraced major party candidates. Both Dems and liberal Republicans (remember those?) sought their endorsement and so they were able to influence their policies. This way, it could operate as an independent coalition but still have the power to influence the Democratic Party and its candidates.