I have a young Smokebush that I bought in 2012. it is about three feet high.
Do I need to prune this bush and if so, when?
The smoke bush (Cotinus coggygria) does benefit from pruning from time to time if it begins to look leggy and unattractive. Your bush is young so it probably has not reached this stage yet. I would not prune until it is well established.
Generally speaking pruning is done to remove diseased, dead or broken branches; to control the size and shape of the plant to match your garden space or to encourage dense growth. Also prune out any stems growing inwards or crossing over each other. For the smoke bush pruning is best done in early spring while the plant is still dormant and before any new growth appears. Bear in mind that blooms grow on wood that is a couple of years old. If you prune you will have to sacrifice the blooms for a season or two. However, blooms and leaves will come back bigger and better than ever. When you remove any of the main stems always cut to the base of the bush.
When the smoke bush is fully mature select strong stems and prune back about 1/3 of the shrub. Always make cuts 1/2 inch above a dormant bud. Space the cuts so that your shrub maintains a good natural shape.
There is a practice (coppicing) where you can give the plant a drastic pruning near to ground level i.e. 2 to 3 buds from the base. This is usually done when the plant needs to be totally rejuvenated. It promotes a very dense bush with numerous branches. Again the fluffy blooms will not appear during this period.
See also this link about pruning a smokebush