Abdullah,
All three are manifestations of ONE God... simple as that, all are God, share the same power and authority but share a different manifestation in wich they do not obstruct each other....
Too quote Tertullianus on this:
"to wit, the Spirit was coordinated with the Father and the Son, and was joined with them in substance, coming from the Father through the Son in some way. The Godhead is three in sequence (who came first), aspect, and manifestation (how they show themselves), but one in quality (they are perfect and complete), substance (whatever it is that makes God god, all three are), power (all three can do anything), and purpose (their aim regarding the created universe is precisely the same). He calls the Spirit "the third name in Divinity, and the third degree of the Divine Majesty"
And God is not witnessing to himself... he is showing his presence to YOU to reafirm what is being said in the gospel... the holy spirit is God overcomming and guiding you !
see it that way... you can ridicule me when i'm silent, but you can't when i'm talking... just because i speak in two different active forms of myself doesn't mean i am two different beings. In that same regard i wouldn't object alltough wouldn't approve either, if you didn't like me when i was silent... but if you don't like me when i am at my best then their is no chance of ever liking me.
best of all to understand this is to read the gospel as a whole, and not taking parts from it, cause that will only confuse you... if you read the gospel in full context it's fully clear to you what the gospel says, without having to explain definitions of trinity or the hypostatic union. I can only say that the christian world (wich is the majority on this planet) understands all this, and if the majority can understand it, it must be that it isn't that hard