It's unclear what is wrong with the three of them. Harris and Chatham road stationary bikes during the session, and Ferguson roamed the sidelines.
The guess on Chatham would be his absence might be related to a foot ailment. He has been battling that the last few years.
Also held out of the practice was TE Chris Baker, who is still on the PUP list. He is currently working on his conditioning with strength coach Sal Alosi. After missing most of the off-season program due to a contract dispute, Baker wasn't in the best of shape when he arrived at camp.