If you use the GCC compiler to execute the following program under Linux, what is the output result?

#include<stdio.h>
int main(){
    int a=5;
    printf("%d %d %d",a++,a++,++a);
    return 0;
}

The answer is as follows:

In LINUX GCC compiler
7 6 8
In TURBO C
7 6 6

I can understand the answer of 7 6 6, but I really can’t understand why the output of LINUX GCC compiler is 7 6 8. Although I know this is undefiend behavior, I still want to know why the results of 7 6 8 appear.
Original question source: Several classic interview questions in C language under Linux