Thus, in the words of the Athanasian Creed: "the Father is God, the Son is God, and the Holy Spirit is God, and yet there are not three Gods but one God." In this Trinity of Persons the Son is begotten of the Father by an eternal generation, and the Holy Spirit proceeds by an eternal procession from the Father and the Son. Yet, notwithstanding this difference as to origin, the Persons are co-eternal and co-equal: all alike are uncreated and omnipotent. This, the Church teaches, is the revelation regarding God's nature which Jesus Christ, the Son of God, came upon earth to deliver to the world: and which she proposes to man as the foundation of her whole dogmatic system.
http://www.newadvent.org/cathen/15047a.htm
TO us there is One God, for the Godhead is One, and all that proceedeth from Him is referred to One, though we believe in Three Persons. For one is not more and another less God; nor is One before and another after; nor are They divided in will or parted in power; nor can you find here any of the qualities of divisible things; but the Godhead is, to speak concisely, undivided in separate Persons; and there is one mingling of Light, as it were of three suns joined to each other. When then we look at the Godhead, or the First Cause, or the Monarchia, that which we conceive is One; but when we look at the Persons in Whom the Godhead dwells, and at Those Who timelessly and with equal glory have their Being [in] the First Cause--there are Three Whom we worship (Theological Orations V.14).
http://www.ewtn.com/faith/teachings/GODB12.HTM
Q. 186. How many persons are there in God?
A. In God there are three Divine persons, really distinct, and equal in all things --the Father, the Son, and the Holy Ghost.
http://www.ourladyswarriors.org/faith/bc3-03.htm
First I'll give you the definition of the Trinity, and then we'll work from there. The Bible teaches that within the nature of the one true God exists three separate and distinct persons: the Father, the Son and the Holy Spirit. They are co-equal in nature and co-eternal.
http://www.evangelicaloutreach.org/trinity.htm
Ryrie writes:
A definition of the Trinity is not easy to construct. Some are done by stating several propositions. Others err on the side either of oneness or threeness. One of the best is Warfield’s: “There is one only and true God, but in the unity of the Godhead there are three coeternal and coequal Persons, the same in substance but distinct in subsistence.”13
http://www.bible.org/page.php?page_id=215