Geneva or Bern?

If you are already in Montreux, no need to stay in Bern or Geneva as well, both are easy day trips from Montreux.
I prefer Bern over Geneva for a day trip. Bern is a better base, it is central. Geneva is the western extremity of Switzerland.

Are these the only places you are visiting in Switzerland? Most people would want to visit the Alps, and stay somewhere in the Alps instead of a city like Bern or Geneva.

I have 3 full days beofe I have to be in Montreux for business. Where would suggest visit? I would like to visit the Alps. What about Interlaken?

Mark and Delena, Interlaken is in a wide valley, between two lakes (hence the name). Most people recommend going further into the side valleys, to one of Wengen, Lauterbrunnen, Mürren or Grindelwald.
This "map" may help you: https://www.zentralbahn.ch/sites/default/files/styles/crop_16_9_1400/public/2016-06/Regional-Pass%20Berner%20Oberland_0.jpg
Red are railways, black are cable cars and yellow are bus routes.
I suggest you do some googling on those 4 names and check out some photos.

If you want to know how this fits in with Montreux (and Geneva and Zürich, I guess you are flying in and out of one of those two), here is a rail map of Switzerland: https://www.sbb.ch/content/dam/sbb/de/pdf/freizeit-ferien/ferien-kurztrips-schweiz/internationale-gaeste/uebersichtskarte_synoptic_map_2017_low.pdf
All the red lines are rail routes, and all have at least one train in each direction per hour.

Interlaken is a sensible idea for linking into travel from Zurich airport and onward to Montreux. It also gives the option of different excursions (like Bern) if the weather isn't good enough to go higher. But if you are flying into Geneva not so good, in fact there would be sense in using Montreux as the base and just taking daytrips higher on either side of the lake.

OK..I think Montreux is a good base for day trips to Bern and the Alps. Since I will be there for week. So, let me change gears. Any suggestions for day trips to France from Geneva?

Annecy is worthwhile and just a short drive away. Take the boat to Yvoire.