If you want to get there around 3 that is good, people are usually there earlier than that. I work for wal mart and no matter how early or how late it is there is going to be a stampede.. and it can get very scary
Most of your supercenter wal marts will be open 24 hours. Yes it can be scary.. I work for walmart and I have seen some crap. Get there relatively early.. It doesn't really matter to be honest they flood us at 5 on the dot usually.
All 24 hour wal marts should be open all night long. they usually put the merchandise down he main isle in grocery. they may shut the store down for an hour to set up the merchandise.