add sys_add_three_integers

kern/syscall/add_three_integers.c

@@ -0,0 +1,8 @@
+#include <types.h>
+#include <lib.h>
+#include <syscall.h>
+
+int sys_add_three_integers(int a, int b, int c, int *ret) {
+  *ret = a + b + c;
+  return 0;
+}