They exist, but are largely irrelevant in the big national elections. Access to the debates would be huge for them, but there is something of an arbitrary policy by the non-partisan "organization for presidential debates" (or whatever they call themselves; but there is a group that specifically organizes these things and sets the rules for each one) that only accepts 3rd party candidates that are polling at ~10 or 15% at x number of days prior to the scheduled debate. This is why it is very rare to see them on the national stage, because they traditionally poll below 10%
It is also because most of them are pretty much Loony Toons which, yes--after this year is a term that needs to be re-assessed. Ross Perot (1992) was the last major third party candidate to get a podium at the debates (and the first in a very long time....but it is important to note that these debates first started in 1956 or so, with the Kennedy/Nixon one being the first that was televised in 1960?, and then abandoned for several elections. These really only started ~1974 or 78 as a consistent part of the election cycle) and managed to claim about 20% of the popular vote in the general election. There have been 3rd party candidates before, but *I think* that 20% was the highest vote count achieved outside of the 2 major party candidates of the respective era.
