When 3 or more char are entered, serial data began to flow from Arduino to pc.
No, it does not. No serial data is sent until you press the Send button or hit the enter key.
Buffer does not reset each time the loop() cycle begin
True. It would be useless if it did.
just when i open for the first time the serial port, this happens.
No. The buffer is cleared because opening the serial port resets the Arduino. The buffer is set up after the reset, empty.
my goodness, i'm sweating...
You wouldn't be here. It's freezing.