Can this ever work?
Depends. Not if you consider yourself a third wheel, really. Open relationships are all about trust and most of the time (from what I've seen) the third is never usually a regular partner and it can complicate things down the line.
I'm sure it can work though and I bet it has for many people. Just a case of knowing and getting what you all want from each other consensually and maturely.
I've known people to have a 3rd wheel then they ran off with them lol best to pick someone with a smaller cock lol
I'm sure they work for some people. You would have to be pretty secure in the relationship though. Could I do it? Nope. He's my husband, I chose him and he chose me. I'm too jealous and I'm sure he would be too.
Long-term I don't think it can, short-term maybe. I think it depends on the communication between all 3 participants and what it is that each of you are looking for.
its on my bucket list lol 
works for us i have huby and we attend swingers clubs when hubby away i do play alone with another male or female and we/ i have been doing this over 10 years its a lifestyle we both love also its something we discussed at length for us it works but its not for everybody
I agree with kitten features in that it probably won't work if you consider yourself a third wheel. My partner and myself are polyamourous, and one of the most important things for this to work for us is that everyone feels special and loved, so that no one feels like they are less important than anyone else. Saying that relationships have different dynamics so it takes a lot of communication of how you feel and what your boundaries are to be happy. Personally I enjoy the freedom and lack of possessiveness that being poly brings, whilst still enjoying being in a loving committed relationship. But if you feel jealous or lesser and that you can't discuss these feelings freely and openly in a supportive environment then it's unlikely to work. Bottom line, if you're unhappy with the dynamic don't force it for the sake of someone else.
I hope this is somewhat helpful
As others have said, it totally depends on the situation and the people involved as to whether it can work :) "Open relationship" means different things to different people, and can range from seeking out casual sex while in a long-term relationship to polyamorous relationships where everyone knows each other, etc.
Personally I'm the happiest I've been in ages because I'm exploring a relationship with someone that isn't my long-term partner. It's the kind of thing that is probably more complicated because you have to take more peoples' feelings and expectations into account, but I feel like I'm being the person that I'm most comfortable being and became very unhappy with monogamy.