how important is the big 3rd?
 

the alt to battery positive that is. I did the 2 grounds and have the cable ready for the alternator one but didn't feel like finishing it (the alternator bolt is kinda hard to get to). But is it really necessary though?

Well, the alt to battery pos u say. It's prety important isn't it? It will stress the alt less if you put in a bigger gage wire when there's serious current draw I guess.
Come one, if you started this, better make it right! :)

upgrading the reduces voltage loss
and has nothing the do with stressing the alternator

The factory wire is sized for the factory alternator. IMO this is the least important of the 3 cables to upgrade if running a factory alt.

well I have a stock 120amp alt and 2 amps that are about 800wrms together. Not sure what the draw is. I didn't really have any issues that required the big 3 but I thought I'd do it anyway as it won't hurt.

Finish what you started. You seem to have the gear, just make the time. Even if the alt is factory you'l probably upgraid it in the future.