This is not the truth of the matter, but one possible interpretation that you are spelling out. If God speaks of Himself in the plural that does not mean automatically that this plural is a trinity.
To say that in Isaiah 6:8 in some way is part of this plurality idea is very misleading as if that would be the case then one could easily interpret that actually anybody (human being, angel) could be part of God's plurality. "Whom shall I send, and who will go for us?" It is not completely clear to what does 'for us' here refers to. To God and His angels, to God and His own plurality, or to God, His angels and His people.
The New Testament does not clarify the relationship among the Godhead. What it does is that it gives even more possibilities of different kind of interpretations among of which one of them is this kind of Trinity idea which you express and in which you follow the tradition dogmatic (that is the Dogma of Establishment Christianity) interpretation of this matter, that is, it is the interpretation that won the battle between different interpretations. To say that some one won the battle does not mean that the winner was right, if this would be the case then you must also think for example that Romney was the best candidate of GOP in 2012. Winning and being correct are not at all the same things, not even in the world of Christian Theology.
I don't say that your interpretation is wrong automatically, but at least I can say that it is not the only possible interpretation.
"Air is the very substance of our freedom, the substance of superhuman joy....aerial joy is freedom."--Gaston Bachelard--
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