I asked the great oracle this question:

https://www.google.com/#q=%22what's+...+terrains+3%22

and received this answer (among others):

https://secure.profantasy.com/products/ft_new.asp

So it's mostly a performance-enhanced version for multicore with a few nice features tacked on (and a huge stack of bug fixes, some of which do nasty things to your worlds). It is rumored that there is a 64-bit unsupported beta version of FT3 that can be obtained from the product's developer if you ask nicely.