So let's say I've scored private security force and the runner is tagged. On my next turn can I use all 3 clicks to do 3 damage? I'm a bit unclear at what points I can perform these kind of actions.
As long as you have scored a private security force, and the runner is tagged, you may use a Click to do one meat damage, in the same way you can use a Click to draw a card, or play a card.
It's perfectly allowed to do nothing but 3 meat damage a turn from then on, if you want to!
Any card that would restrict the number of uses per turn would say so. Cards like Elective Upgrade or MCA Austerity Policy for instance both say to use only once per turn, other cards like Housekeeping or NeoTokyo Grid care about the first time an event happens per turn, so they restrict by only firing that first time. Any card that doesn't have a restriction on use can be used as often as you can legally pay the cost to use it, in this case you need to have clicks and be legally allowed to spend them, so on your turn with no actions still resolving.