I would suggest that you take this one piece at a time. For now just print out on the UNO everything that is received from the Micro. Something like this...
void loop() {
while (Serial.available()) {
Serial.write(Serial.read());
}
}
Once that is working you can begin to think about what type of message format you want to use to pass data.
This is almost certainly not what you want...
while(Serial.available() < 3)
Take a moment to think about what this does.