Supposedly Musk has stated that the legs are actuated by nested, telescoping pistons using high pressure helium, given that the system needs to be ultra light.
Gas actuation requires mechanical latching of each segment of the deployed telescopic cylinder to be rigid, but is lighter than the same volume of hydraulic fluids would when fully extended. Falcon however uses hydraulics (probably driven by pressurized gas) to actuate the Grid Fins, which need to be precise and lock firm at any position.
Gas drive is logical for leg actuation, but why helium? Cold gas thrusters on board already use compressed nitrogen. And helium is “the most difficult gas to seal” due to its very small atom size.
Like others I also think that the deployment speeds of legs are not so critical to worth the hassle to control. However Musk has also said that earlier concepts considered using legs as active aerodynamic surfaces. Grid fins are probably much better for that purpose (faster and need lower actuating force). The speed of legs opening is determined by the force imbalance on them.
The driving force from a pressurized gas tank is a function of displacement. Actuating losses and external aerodynamic loads act against the driving force and are likely to be different on all three legs.
I wonder if one of the first stages that tipped over after landing on the drone ship when one of the legs were collapsing was actually caused by a slow leg opening when the cylinder-stroke-latching cycle hadn't finished yet.