Ibrahim al-Aaraj, who works for the Palestinian Authority's ministry of prisoners' affairs, said the men reported being moved from one ward to another with occasional stints in solitary confinement, Maan news agency reported.
They have been drinking only water and eating nothing since Feb. 26, protesting imprisonment without trial, he said.
Muhammad al-Najjar, one of the three, told the lawyer an intelligence officer at Israel's Ofer Prison, told them they would be fined and family visits would be cut off if they persist.
Najjar said he has lost almost 30 pounds since he began the strike.
Beyonce flaunts bikini body, Blue Ivy in vacation pics
Jordana Brewster on Paul Walker: 'He was an enormous presence in my life'