it used to be that reciprocal links are the ways of SEO masters to amass a voluminous amount of link juice and rank high on search pages--until the time came and the search engine algorithm was changed. It now discredits websites that have a lot of reciprocal links.
Thus came three way links, which are said to be the next way to circumvent the crawlers. I won't be surprised next time if four-way links will come.