Shares of medical on-demand software provider athenahealth fell after the Watertown company missed third-quarter sales estimates. The company reported revenue of $105.9 million, up 26 percent from the year-ago quarter but about $4 million short of analysts’ consensus. Profit rose 25 percent to 30 cents a share, 4 cents above estimates, though that was due to a one-time item. Officials guided full-year revenue at the low end of its previous range of $425 million to $430 million.