There isn't a real obvious difference, but ph3 has much more flexibility, is more streamlined, and has quite a bit more functions 0.12m didn't have. Of course, there isn't anything that you couldn't do in 0.12m, albeit probably with more difficulties than ph3 depending on your goal. Ph3 also doesn't nearly have as many hard coded things that 0.12m had. There are some things that ph3 does not presently have that 0.12m had, however, the website states that they are planning on implementing them eventually.
If you mean the syntax, the framework is slightly different, but the coding is almost identical. Identical enough that you could follow 0.12m tutorials fairly easily, barring the framework and function names (and to an extent, player scripts)