Illinois holds a special election for U.S. House, 18th district, in September 2015. Three Republicans and two Democrats filed petitions to run in the July primaries for this seat. See this story. The deadline was April 20.
It is very unlikely any minor party or independent candidate will run, because they need 12,602 valid signatures, although the deadline for them is not until June 25.
626 Democratic signatures or 1014 Republican signatures or 12,602 third party or independent signatures! Obviously there is discrimination vs. those not of either of the two major parties.