This relatively simple mixer was designed for three dynamic microphones, but can be re-designed for more or less. Level and tone controls are available to tailor the sound to your needs.
R1-R3 are level controls. R9 and R11 control bass and treble, respectively.
Since the circuit draws such low current, two 9V batteries can be used for a power supply.
Building the mixer in a metal case will cut down on noise.