recommendations

The answers to your first and second question can be found on the ole miss greek life website. Each sorroity has their own deadline. These dates may not be chosen yet, but if they are, you can find them on the individual sorority's websites. The links to these websites are again on the ole miss panhellenic website. These two questions are asked constantly. It's very annoying to see over and over when the answers are not hard to find elsewhere. Google is your friend, we are not.

Your third question is a good one for a website like this. Most sororities are already collecting lists of pnms they like. Do you know any girls at ole miss? If so, contact them. Stay with them during spring parties. Meet actives and other pnms. Who is writing your rec letter? Ole miss alumni? Does she have any pull? What does your resume look like? GPA is a 3.0? Community service and involvement? Your social media? Your appearance? I'm not saying you need to be skinny and blonde, because I'm surely not. But do you dress well, hair makeup etc? These are ALL things we look at. Figure out what you need to work on and do it. You still have a few months to prepare and DO YOUR RESEARCH. The Internet can be so helpful for Rush :)

What she said

1) Send AT LEAST one per house. Be safe and send about 2-3 to each house if you can. If you cannot, not the end of the world as long as you sent one.
2) As soon as possible. Don't be that girl who comes to school in the fall and hasn't done it already. You can find the due dates on each sorority's website (they're not updated yet because we don't know the rush schedule yet.... Be safe and send them in by late June or early July)
3) Include everything. Grades, test scores, volunteer work, picture of yourself, teams, clubs, etc. There's no secret ingredient to a "perfect" sorority rec and honestly it's hard to stand out from 1500+ girls going through rush so don't worry about it.
The rec letter is basically "Hey! I'm gonna go through rush! Here's a preview of me!". The time to stand out is during rush week when they can actually meet you.

Letters of support in addition to rec forms?