Why nuclear deterrence was never purely bipolar

Is a three-way nuclear balance inherently less stable than the supposedly dual Cold War system?

The premise is questioned because several nuclear arsenals already existed; this reply does not establish that a three-way balance is inherently more or less stable.

A simple two-power account of nuclear deterrence leaves out Britain, France, and China. India and Pakistan further complicate that picture. The central point here is that several nuclear arsenals already existed, even when the US-Soviet confrontation dominated discussion. The reply challenges the starting assumption of an entirely bipolar system; it does not demonstrate that every arrangement with several powers is equally stable.