Does anyone know please whether it is possible to visit all of these cities in one trip with a reliable but inexpensive operator? Thanks in advance for all replies, good or bad. Doesn't have to be a tour operator within CH.

Sort of a late reply, so hopefully you have found a suitable operator already, but just in case... here's a link to the local tour operator's website http://www.balticvision.com/ as far as I know they do not sell directly to end-customers but you can see a list of agents, albeit none in Switzerland.
Disclaimer: I used to work for another company by the same owner, therefore positively biased about the quality

No, haven't found anyone yet - your's is the first and only response. thanks!

I did all of those cities a couple of years ago but didn't use any operator. What I did was to get a copy of In Your Pocket for each city before I arrived.
http://www.inyourpocket.com/estonia/tallinn
http://www.inyourpocket.com/latvia/riga
http://www.inyourpocket.com/lithuania/vilnius
http://www.inyourpocket.com/russia/st-petersburg
The guides have maps, up to date information on accomodation, sights, day trips.

Travel between those cities usually takes 4 hours (apart from St. Pete). I travelled with Eurolines. The coaches have wifi and are very comfortable and departures are frequent. It allows you to have flexibility for your trip which I personally prefer.
http://www.eurolines.ee/

Have a great trip!

Indeed, one can only agree with Zeirtz above, it's an easy area for independent travel either by bus or rental car, whichever you prefer.

I was merely referring to an operator because of the original question - otherwise, simply go for a do-it-yourself tour, all services are easily bookable online or by e-mail.

Enjoy your trip!