What a choice! I would look at what you do the most, and what meets your needs the best.
I assume that they all work well; if not (be honest with yourself), look at getting rid of the least reliable. It took me two years to convince my mom to trade in her embroidery machine; she wanted to keep it "because it's a good machine", but she hardly used it because the touchscreen didn't work properly, so she couldn't press the buttons necessary to make it work, and it required an external card reader that didn't work with her new computer so she couldn't load new designs. She finally got a new one, and embroiders all the time. I know you love them all, but if there's one that doesn't love you back, it's a good place to start chopping.
Do you have attachments for any that you use, and don't have or can't get for the others?
Do you embroider, and do you think you will continue to to so in the future? If so, keep the embroidery machine; if not, get rid of it because it's the biggest. Check what the building construction and rules are before you decide to keep the embroidery machine. They're loud, and they run much more constantly than regular sewing machines, so they can cause conflict in apartments without good sound separation.
Also, how old is the Baby Lock? If it's old enough that you can't use a USB stick to put patterns on it, I would consider getting rid of it and the Bernina, and getting a new embroidery machine.
Do you travel to sew at other locations? If you do, keep the Janome, since it's the most portable. If you never sew outside the home, get rid of the Janome.
Do you FMQ? If so, keep the one that has the biggest harp/works best for this purpose.
To be honest, if I were you (and assuming the BabyLock is fairly new), I would probably keep it and get rid of the other two. Only keep a second one if it can do something the BabyLock can't (like travel with you to classes). If you're having to downsize substantially, don't keep a "backup" that you will rarely use; it will just clutter up your small home, and get in the way.
Good luck! I hope you find a good home for the machine you decide to let go. Rehoming a machine to someone that needs it can be very rewarding, if you know it will be used and loved.