The Watchtower Bible and Tract society (Jehovah's Witnesses), do not believe in the concept of Trinity. The following is a quote from their website.

Those who believe in the Trinity are not "holding God in accurate knowledge." (Romans 1:28) That verse also says: "God gave them up to a disapproved mental state, to do the things not fitting." Verses 29 to 31 list some of those 'unfitting' things, such as 'murder, strife, being false to agreements, having no natural affection, merciless.' Those very things have been practiced by religions that accept the Trinity.

More on their beliefs of Trinity can be found at the following address:

http://www.watchtower.org/library/ti/index.htm

I suggest that if you decide to read this tract from them you also read the article located at the following link. It shows the improper methods and misrepresentations of the sources that they use in their article.

http://www.macgregorministries.org/jehovahs_witnesses/examtrin/examintrinity.html