Can anyone suggest a really good quality rack mount mixer - to handle two instruments and a microphone?

I have used a few micromixers in the past.  The one that comes to mind is a Samson.  It is no longer being made, and was not a great quality mixer.  The last time I looked at a mixer I came here:  http://www.beyerdynamic.co.uk/feed/small-audio-mixers.htm  but I can't vouch for the quality.

Thanks for that - the BIAMP 301 looks like it might fill the bill.

If the microphone is a dynamic and you don't need separate monitor and main controls, you might try the D-Tar Solstice. It can do 2 mics and 2 lines and sounds way better than most of the cheap mixers.
 
HTH
Edwin