Bernkastel-Kues, Traben-Trarbach and Zell - all doable in one day?

Easy by car. The most beautiful stretch of the river. Impossible by train. Possible by bus but I would not attempt it. Boats also offer tours but I don't know their schedules.

I went by train and bus to Bernkastel-Kues and Zell in 2008. I would not have had time to do Traben-Trarbach too. The travel time is not insignificant, but the main issue was the time I spent in each place - long enough to have a meal in each. I went by bus from Wittlich to Kues. Wittlich is outside the VRM, so only the Rheinland-Pfalz-Ticket would cover all of your travel.

I wasn't interested in T-T, so I went from Kues to Wittlich by bus, from Wittlich to Bullay by train, and from Bullay to Zell by bus. I think there is also a bus route from Kues to Zell via T-T, so you might be able to spend some time in T-T if you did it that way.

But trying to spend quality time in three venues in one day, regardless of travel time, is a stretch.

Problem with the VRM Minigroup is that the validity ends at Bullay on the train, but it is good to Zell from there on the bus. You would need a separate ticket to get to Traben-Trarbach on the train as it is in the Trier verkehrsverbunden, as is Bernkastel-Kues. If you like bicycling, there is a good bike path along the Mosel between B-K and T-T, and you can bring your bike on the train from T-T to Bullay. There are also bike trailers on some of the buses.

We spent almost a whole day in Traben-Trarbach. Besides strolling around the two towns and the bridge, meals, and the bicycle museum (?), we found a trail up through multiple vineyards that ended up at a B&B that also had (paid) wine-tastings from that vineyard and a cafe. We ate by ourselves at the tables on the deck in the sun. Later, we walked/scrambled up to the overlook/fort/memorial on the other side of the river. Are you sure you want to miss these local experiences in order to check off three towns in one day? BTW, don't count on a wine tasting in Bernkastel-Kues without an appointment. Very upper-crust tasting rooms. Many fewer souvenier shops in T-T than in B-K.

As for a wine tasting, I did that in 2008, then realized that it's somewhat pointless unless you are planning to buy a bunch wine from that vineyard to send home. The day before, I tasted three different wines off the menu at a restaurant in Bernkastel, and that worked at least as well. I "tasted" a lot of different wines in the three days I was there (burp).

If you don't have a car and still have questions about the best train tickets, I would ask at the Cochem Tourist Information office (based on Rick's note that the Cochem train station often is not staffed).

Thank you all for your helpful responses. Sounds best to spread this excursion over two days. Also appreciate the tip about the tasting rooms in B-K being on the uppity side. Given that, I may need to adjust my plans.

Jennifer, it is good to seek advice but I would never do or reject something on the basis of one opinion.
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For instance, we stayed at the S.A. Pruem Winery Guesthouse on the edge of Berkastel-Kues a few years ago and participated in a wine tasting there. What we experienced was a small B&B amid a working winery. Nothing upscale about it, other than the other couple (it was mid-March and there were only four of us staying there), who were otherwise good company but seemed pretentious to me with their sniffing and constant sloshing, while we probably seemed uncouth to them.
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Perhaps because there were just four of us and the weather was warm, we sat outside on the patio and the matron poured samples of six or eight kinds and grades of wines for us to sample while she discussed them. In other words, just a normal wine tasting. We did not take a tour but were free to wander the grounds and vineyards at our leisure. My impression was that the place was geared more toward wine enthusiasts rather than tourists, unlike some of the tasting rooms around the Rhein,
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That is just my opinion. It is based on one visit during the off-season. Things might be different at different times of the year. I would recommend that you do some further research, not just about that place but others in the area. In any case, the link for the Pruem winery is https://www.palmbay.com/tours_saprum.asp