you have to look in the cells down in the hospital wing (far left on the map) get through security on ground level, there is a guard sitting next to a computer, there is a key card next to the PC or through the long basement tunnel. You will reach a sort of office with a glas wall, climb through the sort of counter opening there. Then down the stairs to the cell area.

It's at the top floor where they are doing photoshoot. In the office.

They are not agents they are Mafia crooks, but dont matter.
Yes the guy in green is one of the targets, he does the work out routine, WHAIT! until he goes for a walk to the pool outside. Watch him (he does have a drinking prob so whatch where he hides his bottle at the pool. Use ur syringe. Target 2 is a bit mad, sits upstairs and mumbles to himself, just do him with the wire. Ok now the Agent. he is in the Hospital wing. 2 ways to get there. 1: Get to the wing using the underground tunnel (basement)
2: there is a guard room, 1 guy sitting watching a monitor. You can go outside that room trough the window, break the electric box outside so the guard tries to fix it, sneak in and get the keycard on the table next to the pc and go through the gate that leads to the hospital wing. When at the HW sneak through the counter and do the guy sitting there (you might need hes uniform, 2 more guys in the actual ward that could spot you. Or sneak and use the pole to climb down to the ward, this is harder) Now the agent is down there in one of the cells. Talk to him and do the stuff with the syringe etc etc...

I spent many many minutes trying to figure out a way to sabotage the weightlifting, really thought that was the way to kill the bastard