"Wrist-turning pin" or "third form".
Sankyo builds on ikkyo or nikyo (first and second form). It can be understood as a follow-up technique that is used if ikkyo or nikyo do not work, e.g. if uke pulls back from the nikyo grip.
The characteristic element of sankyo is to hold uke's turned wrist as you would hold a sword. Another characteristic of sankyo is the pronounced backwards movement of tori when leading uke into the pin.
The sankyo pin is distinctly different from ikkyo and nikyo pins.