Tom Clancy’s novel is again on the cards, specially if Clinton wins.
But the World War 3 will be fought quite differently than the old US/USSR.
The war would probably start in Syria as a staged accident similar to the shooting of the Russian MIG last year, albeit more serious.
Most likely the West power would stage the accident as Russia would very much prefer to have a war in 3 years time were its army will be fully reconstituted with the T50 PAF kKA 50 5th generation Stealth Fighter and the practically automated T-14 Armata tank.
There is the possibility of a limited war in Syria similar to the Argentine/ UK war over Falkland.
(1) In this case the war would be probably be won by the West as the assets in place of the West are too great for Russia/Iran. Still there are about 4,000 US advisers in Syria that would be at the mercy of Russia (which usually have no mercy at all). Also the Russian anti air systems will take down quite a few NATO aircraft. In that case also you need to watch the Nagorno -Karabakh region (behind Turkey) for a war extension.
Most likely this would be escalated horizontally by Russia in order to make the war too expensive for the West.
(2) Horizontal escalations means hitting adversaries in different locations (closing the straight of Hormuz and Bab al Mandaib Straits (Yemen) choking the oil transport, infiltrating the Baltic States and Ukraine. Possibly, since the rise against the elites in Europe and US, Russia could revive the concept of terrorist organizations like the German Red Army or the Italian Red Brigades which were supported by the KGB). As a side feature this would restart an economic crisis as no Western democracy can now withstand a $150 oil.
This would bring a negotiated peace proposal.
(3) From that “benign” scenario, the next thing is a full blown cyber attack on financial and infrastructure targets in the West that would wreck utter disaster (you will know it when your phone and banking accounts will stop working). This would accompanied by Spetznaz (Russian special forces) on soft target as nuclear power stations.
(4) The fourth scenario includes the above, plus some real confrontation between US and Russia.
Russia will potentially win in the Baltic States and win the Black Sea battle and Turkey, but be stalled in Europe (as described by 16 different US war scenarios ran in 2015 by the US Army) – with the potential loss for NATO of 35,000 soldiers and over USD 15 trillion in equipment). The issue with the Baltic State that, even if there is a considerable more NATO troops than Russians, they would be not able to be deployed in time. In the Black Sea the issue is that the US warship are like duck in a pond for the Russian missile defense system.
Central Europe instead is well defended and the US has also some smart tactical nukes.
This is not considering the potential involvement of China (which in August declared pro Assad) which would open another front against Japan, South Korea, US and Australia as main enemies (Taiwan, Malaysia, Vietnam, Philippines, India as secondaries).
(5th) The fifth scenario logically includes nuclear bombs, tactical small yielding weapon with low level of radiation (1 to 300 kiloton)
(6th) The worst case scenario is the use of the top level nuclear ICBM – the worst being the Russian Topol -M (“Sickle B” as NATO denomination). With a speed of 26,400kmh it needs 22 minutes to deliver 800 Kiloton warhead to New York and vaporize circa 390 square kilometers – a radius of 15 kilometers. After an hour, radiations would take care of the rest.)
And the American would strike back with their similar Minuteman III
My personal forecast that at level 2 NATO would dissolve as country like Italy, France would refuse to participate to a US led war world and also in the US I do not know if the people are ready for an elite war world with 1,000s of casualties. Too many war movies explained too well that there is no honor in war.
In effect from scenario 3 nobody is winning as casualties from both side would cripple the entire world.
Hopefully Clinton and Trump are aware of this scenario and are prepared to negotiate a world were the US is First Among Equal and not the only super power.