Before I go to the possible ABC weapons implementation, first things first. In HoI3, one of the rules is:
Strategic bombing in HoI3 will be abstracted and not allow you to terror bomb civilians specifically. Chemical weapons will also not be included in the game. Any threads that complain about this issue will be closed without discussion.
Obviously, this can't be the case with Cold War game if it turns hot. So, question to the developers - how are you going to change this rule to fit the era?
Now, on the nature of the modern ABC warfare.
In the triad of ABC weapons, biological component is definetly the most capricious one. Due to delayed effects and high chance to affect own troops and population, it's not useful in the classic war operations. Modern frontline troops are decently protected against the chemical and nuclear weapons and this protection is effective against biothreats as well.
There is a reason why it was still considered part of the triad though. While more or less useless (and dangerous to both sides) on the battlefield, it's well suited for the cover-ops deep in the enemy territory in the preparation phase before the conflict. Limited outbreaks in the mayor transport hubs (like Atlantic ports of USA or mayor railroad stations in USSR) could indirectly affect ability of the country to react as effectively as expected.
Bioweapons can be also effective in covert operations aiming for specific people.
Still, there are drawbacks - side employing such weapons loses the suprise effect. Ultimately, due to the incubation period, outbreak will draw attention of the authorities and might warn them.
Role in Cold War game - can be ommited.
Second in the effectiveness in the triad, chemical weapons went a long way from the time they were first employed in Great War. They changed both in lethality (ie. nerve gases), lasting (modern C weapons can degrade fast allowing advancing troops to enter the area) and means of delivery (intruduction of relatively accurate tactical missiles). As such, from 60-ties and later, they were intended by USSR for two roles:
- weapon of terrain denial (using lasting C weapon contaminated area, thus limiting its use for the opponent) - think communication/logistics hubs not far from the frontline and such,
- ability to take over the facility after the attack with infrastructure intact.
Second role was critical for the lasting usefulness of C weapons - first role can be covered by A wepons, second one - not so much (note - neutronium bomb was an attempt to achieve parity here).
Role in Cold War game - depends on choices of devs, but in modern warfare chemical weapon can be used in classic way (on the front) or as a warhead of tactical/operational missiles. Frontline use gets limited though, especially in 60-ties and later due to the ABC protected vehicles and general improvement in protection/decontamination techniques.
King of the triad - tactical and operational nuclear weapons. It's worth noting, that bombs dropped in 1945 on Japan as a part of strategic bombing by present standards are tactical weapons (although in higher tiers of those). In general, tactical and operational nuclear weapons in the doctrine of USA and USSR were intended as the means to augument their armed forces in their main roles - being that stalling USSR in Western Germany or advancing to the Rhine in one week. NATO plans called for mass nuclear attack with operational weapons on the lines of rivers in Poland, separating Soviet forces in Germany from the 2nd wave of units coming the USSR. On the other hand, Soviets planned to use their tactical/operational weapons to deal with any identified concentration of NATO forces that might be a threat to the blitz advance of the Soviet Army.
A such, nuclear weapons (on the level lower then strategic) can be used in 2 main ways:
- as a area denial weapon (think NATO doctrine),
- as a breakthrough weapon (think USSR doctrine).
Role in Cold War game - if the use of nuclear weapons (no matter the level) doesn't mean automatic game over screen, divide between (at least) three groups, tactical, operational and strategic scale weapons is neccessary. Leaving strategic ones off the screen (personally I think the moment they start to fly, it's good time for game over), two other tiers of nuclear weapons are part of warfare of the era as both sides of the conflict imagined it. So, their use should most likely differ from HoI series "whole province" effect - in case of the operational weapons it can be several HoI size provinces, while in case of tactical it's indeed less then 1.