Back to Breast Open turn or Touch turnThe open turn is basically very similar to a one hand touch freestyle open turn with the only difference that one glides to the wall on back. The most important part here is to reach for the wall on a side still slightly leaning on the back and then very quickly bring your legs to your chest. In other words basically you are pivoting on your butt to make this rotation turn. This turn is the easiest to master and when done well can be very effective.
Backwards flip turn or Bucket turn or Rolling turn or Suicide turnThe bucket turn requires a bit more skill, but it basically is just a backward flip with the touch on the wall. The main point to talk about here is that the start of the turn has to be with the palm touch way below the surface of the water. So actually the swimmer has already initiated the turn before the hand touches the wall. After that again tucking your knees is the common element. One problem with this turn is that it requires quite a good lung capacity to execute the breaststroke pull out afterwards, so unless you can hold your breath long enough to not cut the breaststroke pull out short, I would not recommend it. Hence the name "Suicide Turn" I presume :).
Cross-over TurnThis is the newest of the turns. If executed well, it is much faster, so in shorter individual medley distances such as 100 or 200 it would make more sense to learn it. However, if you look at it quickly, you will probably feel confused as to what hand touches the wall and on what side to flip your body to:). No worries, the below video is very good at describing how it is done. I couldn't have explained it better. In short though, after you touch the wall with your upper arm over your body (keep on your back slightly), you will need to drive your butt in the direction below the hand that touches the wall to complete the turn. So in a way it could be performed on a side or as a regular tumble turn depending how coordinated the swimmer is.
Confused? No problem, the pool is yours and with time you will get it :). Maybe the below video will help. It has more detailed explanation with some dryland practice for the open turn and the cross-over turn: