You are playing with undefined behavior. For any single variable, there should be a maximum of one pre/post increment/decrement operator. Otherwise your results may vary: You could just as easily have gotten 1 2 3, 1 3 2, 3 1 2, 2 1 3, or 2 3 1.
The actual reason you are getting 3 2 1 is because your compiler plays with the arguments in the same order they must be pushed onto the stack to call a c-function. However, that's just one compiler. Others could easily give you different results. (Heck, the same compiler could give you different results at different times.)
Therefore, better to say:
int a = 1;
printf( "%d%d%d", a, a+1, a+2 );
a += 3;
This avoids the undefined behavior of multiple pre/post inc/dec ops for a single variable in one statement.