Basically, the logic is pretty simple. The differences between the 3 kingdoms are far greater than the differences within the 3 kingdoms and yet they have been evolving far longer than the period between the hypothetical common ancestor and the emergence of the 3 kingdoms.
I am being lazy and just going to pose a question that I hope someone with more knowledge can answer.
What is the evidence for this statement that the 3 kingdoms have been evolving for longer than the period between the hypothetical common ancestor?
From my VERY rough and hazy memory from school many years back, I seem to recall that we have basic cellular life at 3.5GA and the division of the three kingdoms somewhere at 1.?GA. Even if the ? from the previous is large, doesn't that leave a heck of a long time for a variety of unicellular solutions to develop from some primitive forms?
Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)