I haven't seen any bipods shorter than 6" and wasn't aware they made any. At 3 inches I would imagine you would have to be propped up on something [wall, range bag, box] to be useful unless you were always shooting downwards from a higher position.
I'm curious what situation you would want a bipod shorter than 6 inches? I extend my 6-9" all the way out to 9 when shooting from the bench so I ended up getting a Harris 9-13" as it starts where I like on the bench but allows further extension should I need it myself.