I would say don't mic them. If the arrangement is any good, those three instruments should complement each other nicely and the players will balance.
 

Dear all, thanks for more tips.  My concern here is primarily the overall volume in a full hall - not want it too soft for the people at the opposite end of the hall, nor too loud, certainly don't want feedback from the loudspeakers; the musicians can balance their own relative volumes.  The do-not-mic option is obviously the simplest option, but I honestly do not know how sound-absorbing 100+ people in a fairly full hall with soft chairs and carpet floor will be.
 
Maybe I'll set it up to record, and make it such that the PA operator can route some of it to the main speakers if it really sounds too soft and leave well alone if loud enough - then I can upload it for you all to play spot-the-melodyne...
 
Wei Liang

The best sound will come from no amplification.  I doubt you will have an issue with people hearing it.   I can't be 100% sure, all I can tell you is that I have recorded small groups like this in the vicinity of 500-600 times in all sorts of venues with hundreds of people in the audience and hearing them clearly without amplification has never been an issue.  As long as there is a roof over your head and no leaf-blower parades outside, you should be golden.    
 
Say you just want to try it, (since non "needing" to has never stopped me from doing anything), if you use a single shared omni for the ensemble, it will probably feed back.  

Dear all - in the end we did not amplify this, and it worked out just fine.  Am grateful for the advice given, as it helped me to be more confident about not amplifying in this situation.
 
I did try to record the performance in any case, with an overhead C1000S, pointed toward the cello (or so I thought); and that picked out the violin and the horn well, but the cello - not so well, particularly the lower notes.  Probably with more experience about mic placement, etc I could have done better.
 
Wei Liang