Bandpass Filter Order vs Sample Numbers of Noisy Signal
You are dealing with a time frequency uncertainty in estimation problem.  It takes longer than 3 seconds for there to be enough information in a signal to tell the difference between 3 clearly separated sinewaves between 49 and 51 Hz, and only 1 fat (slightly modulated) one.  And you can't filter out a signal unless there is enough information to tell whether it's even there or not.