You're not supposed to harvest in the first year after planting and even after that you should only pick a small amount from each plant for the first couple of years for them to become established -you should always leave about a third of the stalks on the plant at any one time anyway apparently.
I followed this method when I planted mine and it now grows every year to immense proportions and from 3 crowns I get more rhubarb than I can manage even when giving lots away! My mum however ignored the 'don't pick in the first year' rule and has the smallest, most pathetic rhubarb ever known.