From the WotC website: The format for Mythic Championship III is Traditional Best-of-Three Standard Constructed across all three days of competition. The prize pool for Mythic Championship III is $750,000 in total.
Day 3 – Top 4 Double-Elimination
4 players compete in a true double-elimination tournament. In the grand finals, the winners in the Upper Bracket and Lower Bracket will face off. The player in the Lower Bracket must win two matches to win the tournament!
How to Qualify for Future Events
Players can qualify for Mythic Championships featuring Magic: The Gathering Arena by finishing in the Top 16 of the two-day Mythic Championship Qualifier Weekend.