Champagne and sparkling wine is fairly sensitive to light and temperature but if it's been kept in a dark place, with a constant (fairly cool) temperature there is no reason why your champagne shouldn't still be ok. You don't say whether you have a vintage champagne or not either.
I should also point out that bottles of champagne do NOT have to be kept horizontal, like wine bottles. Wine is stored horizontally so that the corks don't dry out over time, causing shinkage to the cork and oxidation to the wine. The CO2 in the space between the champagne and the base of the cork provides more than enough moisture to keep the cork swollen, and of course standing the bottle up prevents contact of the cork with the wine which can result in corkage.
Bearing in mind you have already kept this bottle of champagne for three years I don't think keeping it for another nine months or so will make much of a difference, especially if it isn't a vintage bottle. Try to store it somewhere dark and cool until your event, but don't be too disappointed if it hasn't kept well.