Background: I am 28 years old living in Ohio, I graduated from a small college and worked as a sales representative for 2 ½ years for a large insurance company on commission but it was still a W-2 position. I worked for a logistics company as an account executive (sales) for 8 months and was let go but I really didn’t like the job nor was I making sales so it wasn’t the end of the world and most employers are willing to forgive one bad job choice as long as you have had a good overall work history. I then worked for an independent commercial insurance firm but I was paid W-2. They paid all expenses, gave me a small salary UNTIL my commissions go to the level that was over that salary amount or 2 years whichever came first. So I assumed that I would have 2 years to make it to a certain level especially since it takes a long time for companies to review their commercial insurance and they only do it once a year.
Last Friday was my third month there and without any warning or letting me know that he wanted to talk to me my boss/ partner in the firm told me that they were letting me go based on my activity as well as my personality (“too laid back”). I am not asking if this was or was not the right fit for me because it wasn’t at the time but I think that it takes time to adapt your personality into a certain sales style here is what I am asking.
Should I list this on my resume? If I don’t what will I say that I did for the last 3 months? If I do how should I explain that I was let go when I am not really sure why (no specifics were given to me). I fear that big companies will not understand that a small company can just let someone go just because they feel like it.
Please give advice on what is the best course of action in this case and let me know if you have any other questions that I may need to explain in more detail. Thanks.

Should I list this on my resume?
Why not? Make a functional resume that highlights your sales ability and other skills you have. Companies worked at will be at the bottom of the resume and of lesser importance. You don't even have to note dates worked.

If I don't what will I say that I did for the last 3 months? If I do how should I explain that I was let go when I am not really sure why (no specifics were given to me). I fear that big companies will not understand that a small company can just let someone go just because they feel like it.
Many companies are "at will" employers now, so no problem with that.

I would feel that this would be a big red flag, should I tell them that I was let go?

I would feel that this would be a big red flag, should I tell them that I was let go?
Why big red flag?

If you list or not list is up to you, if you don't and someone knows or finds out you did work there, then you lied and will be fired, So only you can make that choice

If you list or not list is up to you, if you don't and someone knows or finds out you did work there, then you lied and will be fired, So only you can make that choice

IF I did not discolse this then how would I account for the 3 months of time?

List it but at the bottom of your resume. Are you going to stay in sales?

This isn't the best example, but one I found quickly to show you formatting --

Functional Resume Sample: General Resume Formats | Monster (http://career-advice.monster.com/resumes-cover-letters/resume-samples/sample-of-a-functional-resume/article.aspx)