Bottom feeders thrive.

Three dollar store chains will make up almost half of all the new stores opening up in the United States this year, a reflection of the dollar store sector's outsized growth in the retail industry as other chains close shops or stop building new ones.

About 45% of the 3,597 store openings that large retail chains in the United States have announced so far this year are from Dollar General, Dollar Tree and Family Dollar, according to the latest figures from Coresight Research, a firm that aggregates the numbers from company filings and press releases.