What is the unintentional consequence of 3rd parties?

Answer 1

In elections with two dominant parties, a 3rd party candidate can siphon off votes that might otherwise have gone to someone else. Two recent examples would be Ralph Nader in 2000 and Ross Perot in 1992.

Answer 2

In the US election system, national elections are known for the most part as being ones held between two major national parties. It's safe to say that by the end of the US Civil War, two major parties emerged. These were the Democratic and Republican parties. Every so often a third party will run a candidate. For the most part this party will be formed then die quickly. Because of the two party nature of the American election system, the third party candidate's platform will be closer to one of the two major parties than another. When this happens, supporters of the party closest to the other major party will draw votes away from that major party. With that said, the third party hopes to win the election. The unintended consequences are that the party least affiliated with the third party will win the election as the other two parties will split their votes, thus allowing the party least affiliated with the other two to win the election.

As an aside, in many parliamentary style governments there can be many parties at play. Italy and Israel are nations with multiple parties.