As different parts of Iceland resemble each others, and there is a reason to trace similar origin of them, could we then compare the parts geologically?

This can easily be done, even trying to find new things in one area by referring to what has been found in the other two areas.
Caution: Seeing this, people tend to overreact, trying to point something out which is not relevant. But this is science, with preconditions, calculations and results – and simplifications as well.