Bandpass filter for summing amplifier input
The easiest way is to add a passive low pass filter to each mixer input, eg. like this:-







simulate this circuit &ndash; Schematic created using CircuitLab

This is fine if you only want a soft roll-off (-6dB/octave), but higher order passive filters are difficult to design because they interact with each other. To reduce interaction you have to greatly increase the resistance in each subsequent stage, which quickly leads to impractically high output impedance.

What's 'best' depends on what is most important to you. If you want sharper filtering then an active filter (requiring one op amp per input) would be a good option. For 3 inputs you would only need a single quad op amp IC.