Will this mini dp adapter work?

At that price it's almost certainly a passive adapter; meaning it will count to your two DVI/HDMI monitor limit.
 
Each VGA/DVI/HDMI output needs a separate clock generator; passive adapters don't create an additional one (active adapters do); as a result you're still limited to the two legacy clocks on the GPU itself. They'll "work" to the extent that you can have your two legacy displays connected via HDMI and miniDP-HDMI (instead of HDMI and DVI); but won't let you add a 3rd legacy display.
 
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