Relevant Link Building is a tedious job and you may be able to do it yourself to certain extent.

A reciprocal link is a mutual link between two objects, commonly between two websites in order to ensure mutual traffic. Example: John and Mary have websites. If John's website links to Mary's website, and Mary's website links to Johs's website, the websites are reciprocally linked. 

Even though major search engines frown on this kind of link building, website owners often submit their sites to reciprocal link exchange directories in order to achieve higher rankings in the search engines. You can consider linking as targeted site advertisements.

Three way linking (siteA -> siteB -> siteC -> siteA) is also a kind of reciprocal linking but a special case. This type of linking is becoming increasingly popular since some search engines are giving less consideration to the value of normal reciprocal links.