Maybe this could help:
#include <WinAPI.au3>
$RESULT = _WinAPI_CreateFile(".COM3", 2, 2)
If $RESULT = 0 then
msgbox(16, "", "Port not available, please plug in your device")
else
$tBuffer = DllStructCreate("byte[6]")
_WinAPI_ReadFile($RESULT, DllStructGetPtr($tBuffer), 6, $nBytes)
_WinAPI_CloseHandle($RESULT)
$sText = BinaryToString(DllStructGetData($tBuffer, 1))
msgbox(64, "Reading", "This is the output of 6 bytes from COM3: " & $sText)
EndIf