Originally the rumors were that a tri-turbo set-up would be two traditional turbos and one electric one. Now that we know more about this "electric" turbo, it looks to be an evolution of the turbo that adds electric assist to get it spooling until there is enough exhaust gas to take over. This could allow for the use of bigger turbos without the lag that they are known for.
It seems like this could be used to reduce the number of turbos needed rather than be a supplemental turbo.
It will be interesting to see how these are used and how they perform.