Cistus are mainly the big shrub rockroses, aren't they? I assumed, from the reference to "clumps" that the OP meant Helianthemum, the little ones about 8 inches high, with huge quantities of flowers about 2cm across in shades of white, yellow, red and pink. Same family, so I imagine pruning is much the same.

If it's the little ones we're talking about, there's no point in dead heading. The flowers are short lived, and the plants just produces a succession of them. Dead heading won't encourage the plant to produce any more.

They will certainly carry on for a lot longer than three years, jest spreading and getting larger and larger. I had one for well over 10 years, which had to be removed because we got rid of the bit of wall it was living on.

They are fairly easy from seed.