It looks like those Fast Track monitor mount arms are designed for three of the same kind of monitor, so they wouldn't be well suited for a 20-30-20 monitor arrangement. From their site:

All three monitors are mounted in the same horizontal plane so there is no need to adjust the height of each monitor individually. Due to this feature you must have three of the same monitors.

http://www.fasttracksims.com/triple-monitor-mount-1/

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