add sys_add_three_integers

userland/testbin/add_three_integers/add_three_integers.c

@@ -0,0 +1,10 @@
+#include <unistd.h>
+
+extern int add_three_integers(int a, int b, int c);
+
+int main() {
+  add_three_integers(3, 5, 2);  // should output 10
+  add_three_integers(-5, 3, 1); // should output -1
+
+  return 0;
+}