Three point Calvinists are a cross between Calvinists and Arminians. They hold to three points of Calvinism although not always the same three points.
Some contend that Calvinism is an all or nothing concept and that it is impossible and theologically incorrect to hold to only certain tenants of Calvinism.
Those who hold to some form of both Calvinism and Arminianism believe that they are being open minded and not dogmatic in their approach to theology. They see this as balancing between the sovereignty of God and the free will of man and believe that the Bible clearly teaches both aspects.
The area of Calvinism that is often most in contention is the subject of limited atonement. At the other end of spectrum the most Calvinistic viewpoint, so to say, is the idea of eternal security. Some even consider this to be the only crucial part of Calvinism.