“Holy, holy, holy.” This triple repetition of “holy” is used by some people as evidence for the doctrine of the Trinity, but that is not the case. It is simply a triple repetition for emphasis, as is found in a few other places in the Bible (Jer. 22:29; Rev. 8:13; cp. 2 Sam. 18:33). The fact that the seraphim were speaking back and forth is worth noting. God apparently created many spirit beings that can talk, and thus 1 Cor. 13:1 speaks of the languages of angels.
“The whole earth is full of his glory!” Even in its current fallen state the whole earth is full of the glory of God. In this context, “glory” can also be a reference to power, and the earth shows the power of God in many ways.