how do I add multiple buttons to different pins how would I write that?
Put the pin numbers in an array and iterate through that. When you find that one of the buttons has become pressed, note become pressed not is pressed then use the index number of the button pin to access an array of values to be sent.
As an example, this illustrates what I described above but needs to be changed to slow down the repetition rate (do NOT use delay()) or to detect when a button becomes pressed, see the StateChangeDetection example in the IDE.
void loop()
{
for (int pin = 0; pin < 3; pin++)
{
if (digitalRead(buttonPins[pin]) == LOW)
{
Serial.println(letters[pin]);
}
}
}