Does true wireless 3-way switch exist?
You can get pretty close.  

"...looks like a wall switch but without wires behind..." Assuming here that your looking for the standard TOGGLE light switch, then you're not likely going to find the exact product.   

If Decora or paddle style wall switch is acceptable, then replace both existing 3 way ends with this or this (the difference is the first is on/off only, the second also Dims- choose the dimmer if the hall light is dimming compatible).

For the middle location choose this wireless switch plus a mounting bracket and any standard Decora or paddle switch wall plate.

Initial set-up (specifically linking) is slightly confusing if you've never before used Insteon products, wiring is however straight-forward.  Insteon (also known as smarthome.com) does have good phone support (although their phone system produces horrible call quality-if that happens ask the support person to call you back as calls in that direction don't seem to be affected).

You also need a standard USB cable and perhaps an adapter to charge the wireless switch. (the middle switch will only need charging a few times a year). Honestly you likely already have this standard USB cable, it's pretty standard and the same used to connect most camera's and non-apple phones to a computer or charger.   (I put smarthome.com links for the last two because those two items appear to cost much more when purchased via amazon.)

Disclaimer: I do NOT work for Insteon/Smarthome.com. I do work for a company that has a sales/installation agreement with Instean/Smarthome.