I don’t usually jump in on things, but you asked one question that few people seem to be responding to, which is whether or not you are to blame. The answer, technically, is “NO” but I think what you’re really needing to find is a way to avoid this occuring yet again. Three times with three different people sounds to me as though you are frequently finding yourself in situations where someone who wants to take advantage will. Some people will call it victim blaming when suggestions for avoidance are made. I am not blaming you for your past, I am giving you suggestions to be more proactive in your future.
1. Everything that happens appears to be either alcohol or “mysterious forgetfulness” related. Which means that these people are slipping things into your drink, most likely. If you go somewhere where you will be alone with a man, either bring your own drink (non alcoholic) or keep a very steady eye on whatever you get. Don’t let them fix it in another room, don’t walk away from it for the bathroom, etc.
2. Learn to say “NO”. Say it loud. Walk away. You can’t do this if you haven’t been following #1, so make sure you do.
3. Rethink who your friends are. why have you been around so many people who would do something like this? Is there a common personality trait you are attracted to that you need to watch out for? I am NOT saying it’s your fault for making these friends, but I AM saying you need friends who aren’t the type to take advantage of someone, or worse. I have never been unable to be alone with any of my guy friends. I can’t think of a one who would dream of doing something like this to a woman. I’m assuming it’s because of certain personality characteristics that I seek in the people I spend time with.
4. If you have been drinking, make sure beforehand that one of your closest (female) friends will stay with you at all times. A true friend will understand that this is a support you need right now to feel/be safe. If you will be alone with someone, such as on a date, make sure someone knows where you are and gives you a check-in call once in awhile. If they call and you don’t answer, they can immediately assume you need assistance.
I am basing the above on the common factors I saw in your descriptios – alcohol/unknown, being alone with a man, all either close friends or friends of friends.
I also suggest therapy to help you work through this, build confidence and better understand your personal decisions and those of others (such as who can/can’t be trusted). There is nothing worse IMO, than refusing to be proactive because someone else shouldn’t be doing something in the first place. Many people equate this with victim blaming, but I find that to be a ridiculous perspective, personally.