It's sometimes used by Christians too, to represent the Holy Trinity of Father, Son and Holy Spirit (sometimes it's adapted by Christians into three interlocked fish). Plus, others believe it represents similar trinities, such as:
* Mind, body and soul
* Earth, sea and sky
* Physical, mental and spiritual
The unbroken circle represents eternity and the number three is often seen as being a divine number. So it could be a good symbol to give to someone whom you wish to be with for eternity or to have around if you want to strength your connection with the divine.