Depends on type of PGMs. You're talking IR, laser or electro-optically guided missiles, or ATGMs if air defense or ambushes are not kept away. The common GPS guided bombs and missiles are very hard to use on moving targets, practically impossible unless they do stupid shit, like moving on a very predictable route with constant speed or crowding near a river crossing for a long time.Well, of course it is. PGMs mean that any sort of movement in the open is incredibly risky, and modern weapons are so expensive that you'll be running out of them in short order.
Only if it's one of those "fuck NATO, we won't spend 2% GDP on defense or even close" countries, obviously different countries maintain different stockpiles of munitions. Some are... definitely insufficient like that. Others, not so much.In any sort of World War 3 scenario, you'd have two weeks of modern warfare at best and then it would be back to World War 1 conditions: unguided munitions, infantry-centric armies and so on. And that is assuming nukes don't fly at all.