The One and the Many: Trinity in Unity in the Bible

"They came to the other side of the sea, to the country of the Gerasenes. And when Jesus had stepped out of the boat, immediately there met Him out of the tombs a man with an unclean spirit. He lived among the tombs. And no one could bind him anymore, not even with a chain, for he had often been bound with shackles and chains, but he wrenched the chains apart, and he broke the shackles in pieces. No one had the strength to subdue him. Night and day among the tombs and on the mountains he was always crying out and cutting himself with stones. And when he saw Jesus from afar, he ran and fell down before Him. And crying out with a loud voice, he said, 'What have You to do with me, Jesus, Son of the Most High God? I adjure You by God, do not torment me.' For He was saying to him, 'Come out of the man, you unclean spirit!' And Jesus asked him, 'What is your name?' He replied, 'My name is Legion, for we are many.'"
- Mark 5:1-9

When I interact with anti-Trinitarians, whether of the Arian or Sabellian varieties, one strategy they all seem to try is to describe the Trinity in this way: "One plus one plus one equals three, so you believe in three gods." For some inconceivable reason, they think this is a very clever argument, even though math has nothing to do with it. Or, if you want to do math, why can't it be one times one times one equals one? Or one cubed is one? If you see what I mean, the argument is nowhere nearly as clever as they think it is. We talk about things as unities in one sense but manifold in a different sense all of the time. Have you heard of the three branches of the one federal government?

Yet, the anti-Trinitarian desperately holds on to this argument. Their answer is, "Well, we do that, but the Bible doesn't."

Really? I guess anti-Trinitarians don't read Mark 5. Notice the interaction between Jesus and the demoniac. Actually the demon in the demoniac. Notice that Mark refers to the demon consistently as "he," not "they." And notice that the demons say "me," not "we," except one time in verse 9. Even in verse 9, the demon says "my name is," not "our names are."

My point is that the Bible certainly does refer to things as one in one sense and manifold in another. In this case, we see one demon also described as a legion, a unit of Roman military consisting of about 6,000 soldiers. If the demon of Mark 5 can be 6,000 demons described as a unity, then why can't the three Persons of the Trinity be one God?