Why it is called the triple Gems?In Buddhism, the following three are called Gems (Ratna) as they are invaluable :
Buddha (The Enlightened One)who, depending on one's interpretation, can mean the Historical Buddha, Shakyamuni, or the Buddha nature or ideal within all beings;
Dharma (The TeachingÂ), which is the Teachings of the Buddha.
Sangha (The CommunityÂ), The Community of those great people who have attained Enlightenment. so that those people(Sangha) will help you to attain Enlightenment.
The three gems are so called since amongst all gems, the Buddha gem and Dharma gem are considered incomparable in value as they are not material, so cannot be created, destroyed or changed in any way. Buddha's mind in his earth body or sambhogakaya is frequently associated with the greatest gem of all, the diamond. In the Anguttara Nikaya(3:25), Buddha talks about the diamond mind:
These three types of persons are found in the world: 1)One with a mind like an open sore; 2) one with a mind like a flash of lightning; 3)one with a mind like a diamond.
1)One who is irascible and very irritable, displaying anger, hatred and sulkiness; such a one is said to be a person with a mind like an open sore.
2)One who understands the Four Noble Truths correctly is said to have a mind like a flash of lightning.
3)One who has destroyed the mind-intoxicating defilements and realized the liberation of mind and the liberation by knowledge is said to have a mind like a diamond.
With this we understand that to take refuge in the Buddha is to take refuge in the mind like a diamond, the hardest natural substance that can cut through all delusion.
The Three Gems when used in the process of taking refuge, become the Three Refuges.
The expression Three Gems are found in the earliest Buddhist literature of the Pali Canon, besides other works there is one sutta in the Sutta-nipata, called the Ratana-sutta[3] which contains a series of verses on the Jewels in the Buddha, Dharma, and Sangha. In the Ratana-sutta, all the qualities of the Sangha mentioned are attributes of the Buddha's enlightened disciples.
The Three Refuges occur very frequently in the ancient Buddhist Texts, and here the Sangha is used more broadly to refer to either the Sangha of Bhikkhus, or the Sangha of Bhikkhunis.
"I go to Master Gotama for refuge and to the Dhamma, and to the Sangha of Bhikkhus."/\
source
:http://en.wikipedia.org/wiki/Triple_Gem