OLD ANSWER, READ THE OTHER ONE
Are you perhaps
When it is just me I have 3
Bit has three letters.
Yet when there are more, I have four
Its plural is bits or can even be byte if there are eight.
My first two are double
The prefix bi- means "twice" or "two".
My last two can clarify
IT is Information Technology, the study of using computers for research, or send and transmit information.
Leave my middle one alone, though, and soon you'll be talking to yourself
I, that's me!
You'll know you have me when all of me points right back to you
Not sure about this one either, but there are bits everywhere on the device I'm using.