Parties come and go, but there will be two major parties. In fact under such as system if you're in one of those two big parties and you see a third party rising you need to figure out whether it's you or the other guys getting replaced, 'cos it won't stay a three party system for long.
For example in the UK, the Tories ("Conservative and Unionist Party") and Labour are currently the biggest parties, but a hundred years ago this was a novel situation, Labour were seen as a third party, while the Liberal party (which was eventually absorbed into what is today "Liberal Democrats") had seen success over decades and were often in government prior to that point.
