Combining 1, 2 or 3 audio signals so the output always has the same amplitude. LM3900
What you need is an Automatic Gain Control (AGC) to keep the output level constant regardless of input level.  A possible candidate is the Maxim MAX9814 which combines a microphone preamplifier and an AGC function with 20 dB of range. Since you have 3 inputs, which presumably have about the same level, then the maximum input level change is from 1 input active to all 3 inputs active or a factor of 3 which is about 10 dB.  Thus the MAX9814 should be easily able to handle this range.