Oh yea. But I can imagine it would be very complex and very difficult. But of course they can. It's one of the reasons why when you are with someone, you don't allow yourself to get involved with someone else you feel attracted to. Mixed feelings and emotions get carried away, and you lock yourself in a really nasty position. You can not only hurt others by allowing yourself to be in such a situation, but you're also hurting yourself and cheating yourself.
I highly recommend, if you find yourself in a situation like that, you think things over really fast, and decide what is better for yourself and the other two, because ultimately things are only going to get ugly if you don't. You can only be with one person. It's pointless to allow yourself to be in love with two at once.
* i was bored again and have nothing much to do so i decided to answer this question that i saw in one of a website that i am surfing right now...