What they said. The Protank 2 and 3 series are nearly identical, the only real difference being the 3's use a taller dual coil that they're designed to fit. The 2's were designed for the smaller single coil so they won't fit the dual coils from the new Kanger tanks, including the Protank 3, Mini Protank 3, Aerotank and Evod 2.
But the old single coils will fit in the Protank 3, there will just be some extra space between the rubber seal on the coil head and the bottom of the chimney. It still works fine if you're just sitting at your desk or something, but it can cause some leaking once you're out and about and it's moving around, left upside down, etc.