NEW YORK (Reuters) - Three prominent investment firms bet changes are in the offing at ailing healthcare giant CVS Health ( CVS ) and added to existing bets and established a new position during the third quarter, according to regulatory filings.

Longtime investor Glenview Capital Management bought 2.8 million CVS shares during the third quarter, increasing its stake by 31%. The firm owned 0.95% of the company on Sept. 30, the filing shows.

Sachem Head Capital Management, which built its CVS position during the second quarter, bought over 1 million shares during the three months ended on Sept. 30, increasing its position by 44%. CVS is now the hedge fund's second-largest holding. Sachem Head, run by former Pershing Square Capital Management partner Scott Ferguson, owned 0.28% of CVS at the end of the third quarter.

Third Point, run by billionaire investor Daniel Loeb, built a new position in CVS by buying 1.6 million shares. It owned 0.13% of the company at the end of September.

All bought shares before CVS in October tapped David Joyner to replace Karen Lynch as CEO after the stockprice had tumbled amid ever worsening financial forecasts. Shares have dropped 32% since January.

Investors became more agitated over the summer and Glenview's Larry Robbins, who likes to be called a constructivist shareholder, recently met with executives to push for changes. He did not call for the company to break up.

Analysts had speculated that other investors might soon move in and make noisy demands for changes, including replacing Lynch as CEO.

So far neither Third Point nor Sachem Head have made any public demands for CVS.

Investors are required to make the so-called 13-F filings with the U.S. Securities and Exchange Commission to show what they owned in U.S. companies' stock at the end of the quarter.

While the filings are backward looking, they are closely tracked for hints on which companies activist investors may target.