If stored properly, Champagne can keep all its qualities for much more than 3 years. Sometime back, somebody found a bottle of Clicquot Rose from the beginning of the century and the Champagne was outstanding, So three years should not be a problem. The ideal storage conditions are at a constant temperature of 12-13 degrees, In a dark place with sufficient humidity. If the ideal temperature cannot be reached (Which is the case most of the time in India), make sure that at least the temperature is consistent and as low as possible. Wines hate temperature variations and it is better to keep them throughout the year at 22 than going from 15 to 30 all the time. Wines need to be stored horizontally in order to keep the cork moist. If it dries, it loses its sealing capacity and lets air inside the bottle, which leads to "oxidation" and leaking bottles.
For Champagne it is very important that you store the bottles in the dark, as sparkling wines tend to develop a very unpleasant taste, called "Light struck", if they are exposed to light for too long. Unlike still wines (Non sparkling), Champagne can be stored vertically because the pressure in the bottle ensures a proper humidification of the cork anyway.