I play skyrim in pc but i want autosave to save on the same file because i don't like having over 100 saves and i have 3 different character accounts i play on and it gets confusing trying to find the right save for the right account and it takes up lots of memory space too.
What I mean is that every time it autosaves i don't want the autosave to be created on a new one,

Autosave will only save over the oldest Autosave. Nothing you can do (unless there is a mod that can do it).

It only creates 3 autosaves afaik. It automatically overwrites the oldest of the 3.

There are save managers you can download so you can separate character saves. I dont use them personally, but I've seen them at skyrimnexus.com

I never have any more than two characters going at a time, and I'm a compulsive manual saver. I have been since Morrowind, so these kinds of save problems have never effected me.