Have you tried such resources as Meetup? In some areas I have been able to find MeetUps set up for all types of Buddhists - or even inclusive of many different types. This provides a way to become exposed to various groupings. Also, many of the Vipassana groups such as Insight Meditation do not require you to join their tradition. I am made to feel welcome even though I am a member of Rissho Kosei-kai. You might try some of the sangha and meditation group listings for Insight Meditation and for Thich Nhat Hanh's tradition (I think he has a listing of groups on his Deer Park Monastery website). Another resource is through local Unitarian Universalist churches and fellowships - many of them have their own Buddhist sanghas and meetings (sometimes affiliated with the Unitarian Universalist Buddhist Fellowship which is inclusive of all traditions).

In my case, I live outside a small rural town with an actual variety of sanghas - a Korean Zen Temple and retreat center, a Tibetan retreat center, a Soka Gakkai local sangha, and an hour in either direction there are independent Zen sanghas and Vipassana sitting groups, but I felt a connection with a tradition which is not represented up here. Fortunately Rissho Kosei-kai is so inclusive that I can associate with the other Buddhists in the area if I wish) so I affiliated through the organization in Los Angeles. Have you looked at the sangha directory in Tricycle or Buddhadharma magazines (or some of the directories online)? You might be surprised what exists near your area.

Fortunately members here come from very different traditions so debates are really interesting and intense. I think it is healthy to share different views about our practice than just become tightly attached to a single perspective. Personally I get bored being around people where all think alike.

Does one necessarily have to belong to one of the three main traditions?

No - and I think that beginners should study the teachings of the historical Buddha carefully first, beginning with The Four Noble Truths which are core teachings.