Too complicated for my little head.
But does it disable only the PA03 functions? Or what else?
I do not see bit 3 in the code.
OK; I actually ran this through the compiler this time. And you're right - it was missing anything about bit3!
It should be:
PORT->Group[PORTA].PINCFG[3].bit.PMUXEN = 0;
At least, that compiles. It does seem horribly complex, doesn't it? Fortunately, everything resolves to constants at compile time, so it doesn't actually generate much code...