import com.atlassian.jira.component.ComponentAccessor
def customFieldManager = ComponentAccessor.getCustomFieldManager()
def dc = getFieldById("customfield_17120")
def dv = getFieldById("customfield_17109")
def cl = getFieldById("customfield_17110")
def dcval = dc.getValue()
def dvval = dv.getValue()
def clval = cl.getValue()
def Sum = dvval + dcval; //calculate
if (dvval != "" && clval != "" ) {dc.setFormValue(Sum)}