Melt 3 tablespoons butter in heavy large saucepan over medium-high heat. Add leeks and potatoes; sauté until leeks are tender, about 4 minutes. Add chicken broth. Bring to simmer. Reduce heat to medium-low. Cover; simmer until potatoes are tender, stirring occasionally, about 20 minutes. Remove from heat. Add chopped watercress. Cover; let stand until watercress wilts, about 5 minutes.