Yep, March 1st would already count as your first weekend and the 14/15th would be the 3rd weekend.
If ever in doubt, make sure to search for specific locations you're interested in - for example, for the Lucca antiques market, I searched for "mercato marzo lucca" and found the town's event calendar page where I was able to confirm the market is that weekend:
http://www.turismo.lucca.it/it/eventi/mercato-antiquario-8