I have done a lot of things in the auto electric business but one thung I have never seen is someone connecting three alternators in series. My first response is that it will not work. They are not batteries. Putting them in series will accomplish nothing so scrap that idea.
My next thought is, why would someone want to do this? There is not enough room for 3 alternators under most hoods, the heat generation would be immense, the load on the motor high and depending on size of motor, probably stall it when under load.
so perhaps we can start back at the beginning and you can tell us what it is you are trying to achieve and why? Then we can begin to help you. Year, make, model and what you want to do.
Otherwise go to page 3 and you will find an article from skipperdan on building a 60 volt alternator. Same deal but his has a story.

He explained exactly what he wanted to do so i assume since you have all those years working on cars you also understand how to read Asking for the year make and model sounds like u work at autozone which would explain your answer because they are helpless with out year make and model not understanding somethings are universal and his ? Refers to a universal electrical system so there are vehicles out there with 2 alternators on them still running today Ford f350 comes to mind or an ambulance i am actually researching this my self and i cant say at this point if they will conflict with each other or not but u can increase the voltage on one alternator or find a belt driven DC generator to charge your battery bank

 

Without knowing the load and circuit configuration it's hard to comment.

But... If you were to configure 3 separate, isolated, 3-phase rectifiers and filter capacitor circuits, you could theoretically connect them in series.

This avoids the nasty phasing problems, it's just DC then.

 

So, you have an 84V battery pack.
Do you have an existing 84V battery pack charger (I have to assume yes).
What is the input to this charger (I will assume 120V).
How fast charge/current required? (you haven't said).
Why not just use 12V to 120V Inverter to power your existing charger?
Custom build/Frankenstein is never easier or cheaper.