I don't know if this is what you need to know, but I will tell you what I do:
If I have a book or document that I want on my Kindle, I convert it to mobi format (with Calibre) and then send it to my kindle at the "free" email address.
I have not done it yet, but I suppose that if I had a document that I could not convert, then I might email it to the "non-free" address and see if Amazon could do the appropriate conversion (for a fee) and send it to me.
Be it right or wrong, that's what I do and it works fine for me.
BTW: sideload via USB means to plug your K3 to your computer via the USB cable and do the transfers that way.