I have an interview recorded using a Zoom H1n audio recorder and and two DSLR internal mics. Is there a way that I can use all three tracks in some sort of noise reduction process that looks for the loudest common sound in all the recordings (the interviewee speaking)? Something similar to adaptive noise reduction using multiple microphones that is carried out by modern phones?
There probably is not much of a point trying to combine the sources. I'd use just the H1n sound while using the DSLR sound as reference for synchronisation with the video (I assume you recorded video on the DSLR). If you have equal quality in all sources, Wiener filtering would give you about 3dB gain for every doubling of the relevant sources but you don't have equal quality. If you work with non-linear filtering like spectral subtraction, you can recover significant amount of recognizable speech at the cost of a weird noise structure ("musical noise"). If you hand-placed a H1n in an interview environment, I would be surprised if the gains from such a procedure would offset the artifacts they buy you.