Bulbs are classified on the basis of voltage as a measure of the current that has to be passed through the filament in the bulb. The current generates heat and if the heat is enough to raise the temperature of the filament to a level that allows visible light to be radiated, the bulb glows.
Any voltage can be applied across the filament of a bulb with a given voltage rating. If a smaller voltage is applied the filament is not heated to a temperature high enough to radiate visible light. On the other hand, the application of a higher voltage can result in the filament getting burnt out and damage the bulb.
On applying a voltage of 3 volts across a bulb which is rated 1.5 V, it will give off light but there is a very high probability that the filament of the bulb cannot withstand the high voltage and will get permanently damaged.