This question doesn't make sense to me: if you are in a polyamorous relationship with someone who has two other partners, you have one partner who has two other partners. You don't have "1/3 of a partner". You also have the ability to find other partners, so you have "more" partners than you would if you were in a monogamous relationship with, say, a very busy person.

Whether this kind of relationship is better than none depends on how it goes for you: if your partner is helpful and supportive and you are ok with non-monogamy, then it probably is. I don't see how this is a "bad deal" at all.