The model of the handset itself needs to be KX-TGA850, and the base (mother) station can handle a maximum of 6 handsets, so adding a third wont be a problem.
The following are the instructions for registering an additional handset once it arrives
On the Handset: Press the centre navigator key), then press # 1 3 0. Then using the Up and Down navigation keys, select the number "3" on the display.
On the base unit: Press and hold the button x for about 5 seconds, until a tone sounds.
If all registered handsets start ringing, press the same button to stop. Then repeat this step. The next step must be completed within 90 seconds.
Back on the handset: Wait until "Base PIN" is displayed, and enter 0 0 0 0
Should you need any further assistance please let me know