The assertion states that if the filament of a light bulb is not uniform, then its life is shortened. This means that if the filament is not consistently shaped or evenly distributed, the light bulb will not last as long.

The reason provided states that the resistance of a glowing light bulb is less than that of the bulb at room temperature. This implies that when the light bulb is functioning and producing light, its resistance is lower compared to when it is turned off and at room temperature.

Looking at both the assertion and the reason, we can see that both statements are individually true. It is true that a non-uniform filament can decrease the lifespan of a light bulb, and it is also true that the resistance of a glowing light bulb is less than that of a light bulb at room temperature.

However, the reason does not provide a correct explanation of the assertion. The reason talks about resistance, but it does not directly address how a non-uniform filament affects the life of a light bulb.

Therefore, the correct answer is option 3: A is true, but R is false.