storing incoming data from BT to a char string

Projects Programming
9 
void loop(){
  Serial.readBytes(buf, 3);

"readBytes" returns an important item of information, but you discard it.
Why?
See reply #4.

 buf[i] = NULL;

NULL != '\0'
If you're going to squeeze a pointer into a "char", it is usually polite to tell the compiler it's a legitimate thing to do.
Or, better still, don't do it at all.
See reply #4

In a simple code like this, the time I take to type the characters does not matter.

It's nothing to do with the complexity, or otherwise, of your code, it is simply how the serial monitor works.