Concatenate integers as string
If you're just outputting to serial you don't need to concatenate - just print each bit separately, such as:

Serial.print(out1);
Serial.print(",");
Serial.print(out2);
Serial.print(",");
Serial.println(out3);


You should avoid using String as it's not good on devices with small amounts of memory like Arduinos. Instead if you really need to concatenate numbers you should use a C string and maybe snprintf to format the numbers into it:

char out[30];
snprintf(out, 30, "%d,%d,%d", out1, out2, out3);