When? In cruise? Check the N1's and you'll have a good reference for an unreliable airspeed event.
On final for power? With auto thrust off(please do!!)? F3 is typically about 5% less N1 vs F Full. Obvious it changes with winds and a/c weight. Try about 50% N1 for F3 and adjust as necessary(it's called pilotage). Or start with 55% N1 for Full flaps and adjust as necessary. Play the constant mind game - why did the speed change? Updraft? Downdraft? Wind change? Is yes how much should I adjust my new target/base line N1? If there were no changes and the speed is decreasing or increasing the initial baseline/target N1 needs to be adjusted. This is what makes flying a skill, and fun!