What is the best Old Testament evidence that God is triune?

The Spirit is all over the Old Testament: in Exodus the Spirit of God fills Bezalel to do his work, and from the first verses of Genesis the Spirit of God moves over the waters, though there ruach can mean wind and is translated Spirit for contextual reasons. Any reference to the Spirit of God is the third person of the Trinity. References to the second person are subtler; the clearest is Psalm 110, 'The Lord said to my Lord, sit until I make thy enemies thy footstool,' which Jesus uses in Matthew 22 to stump the Pharisees, asking how the Messiah can be David's Lord if He came from David. There is also Daniel 9:26, which calls Him the Messiah and gives the timetable: add the 490 years of verse 24 to the decree to return from Babylon, about 458 BC, and you arrive at the baptism of Jesus in AD 29. The Old Testament never gives His name, Jesus, which is why the Bereans in Acts 17 had to be told that this Messiah, this Yeshua of prophecy, was the Jesus who had just died on the cross. The Father imagery, meanwhile, is very strong, in about a dozen places.