Why not dismiss the trinitarian belief outright

[arewethereyet]

You will get more insight into the answer to your question from someone who believes in the concept of the trinity.  Most of us left such faith practices which in part means that we don't buy into the concept.  That shouldn't be difficult to understand.  

Why do people believe what they believe?  You have to ask them.

But I will tell you that William Lane Craig isn't the ultimate source for such things as he is pretty fringe.  Find yourself a nice middle of the road Christian to explain why they believe what they believe with regard to the trinity.

Thanks for the answer. I am not really looking for personal reasons to believe in Trinity, but for real objective justifications of this idea. This is why I prefer atheist forums, at least I won't get lengthy quotes from the Bible as answers. Besides, I thought that the "Christianity" subforum is visited by actual christians but apparently this isn't the case. 

I recently found a video of some christian apologist named Matt Slick purportedly destroying Matt Dillahunty, I am not sure if anyone here knows the former and can advise me to read more about his defenses of christian doctrine.

I am not familiar with Matt Slick...I have little use for apologists.
I suppose the best way to look at it (the concept of the trinity) is the same way many things in religion have to be taken and that is on faith.  That's what I was taught.  You don't ask a lot of questions but instead take the teachings on faith.  
That never worked for me.  I needed proof or at least something that made sense.  Why should I take anything on faith?
Seriously, find a believer and ask them to explain it.  In fact, find a few.  I just went through a Google search to see what's out there for an explanation and I got similar but different explanations on about a half dozen sites.  I think you will find the same is true if you ask a half dozen believers...you will get similar but different answers.  I have a hunch that even those who profess to be believers don't all buy the trinity thing but claim to as it's expected of them.