Does it work if you replace your loop() function with this:
void keyPressed()
{
int x = keyboard.getOemKey();
if (x == 58) lcd.print("F1 is ptressed");
if (x == 59) lcd.print("F2 is ptressed");
if (x == 60) lcd.print("F3 is ptressed");
}
void loop()
{
// Process USB tasks
usb.Task();
}