Well RedTent, indeed there has ever been but one plan of salvation in the Bible, only some things are dispensational. Now since you don't live in the Garden of Eden, going around naked, being a gardener, being sinless; how can you say there was no first dispensation? Surely you don't think you are obligated to be a gardener.
The fact is that some rules in the Bible were given to only certain people and only for certain time periods, in contrast with eternal principles of righteousness. You cannot be a Christian without doing dispensational interpretation, or you shall be wanting to offer animal sacrifices and observe the 7th year sabbath. You will be building an ark because Noah was told to do so. There definitely is a Dispensation of the Law of Moses that ended at the cross. That is not to say that in the Law there are not eternal principles of righteousness; indeed the Law has the commands to love the Lord with all the might and the neighbor as self. In Matthew 10 there are dispensational rules for missionaries, which I doubt many missionaries would follow today. If you own more than 1 coat, you need to come up with a dispensational approach to the gospel teaching about 2 coats.
BTW, you also said that God manifests Himself three ways. If that is all the Trinity means to someone, then that someone is a modalist, a heretic who as denied the Trinity. God has 3 egos or persons, not just 3 modes of operation.