danube cruise worth it?

Presumably the cruise will cover the Wachau Valley section of the Danube, outside Vienna, which is considered the most scenic section. If you "never" plan to do a multi day Danube river cruise, then it may be worth it. But it would take at least 1/2 day out of your 3 days. May also be a "cooler" respite if the weather is hot in the city?

Hey, I'm also planning a 3 day Vienna trip this summer and looking for recommendations. Commenting here so I can get a view of all the replies, thanks! Hope you have a great vacation!

I would definitely want to know where the included tour cruise started from and went, just so your expectations are correct. It might be one of these:
https://ddsg-blue-danube.at/

They have cruises from a few hours to day trips to other cities. We did the short cruise around the "green/south" part of town starting from the midpoint on the canal (Schwedenplatz metro station) and going around the south way to the Danube river and up the Danube thru the locks which I found very interesting as an engineer. It ended on Danube at St Francis of Assisi Church (Vorgartenstraße Metro station). We enjoyed it but it may not be what you are wanting.

Do you mean the train to Melk, cruise the Danube and train back to Vienna daytrip? That is quite well known and well thought of, OBB even has a ticket just for that. Read up on it in Rick's guidebook.

Wachau Valley is a strip of about 20 miles along the Danube river. Going by boat from Vienna and back is a waste of time as the trip between Vienna and Wachau Valley is not scenic.

I have a feeling that this is not a Wachau Valley Danube cruise. The only tour I found that matches the OP's description is this one: https://www.viator.com/en-GB/tours/Vienna/Vienna-Combo-Danube-River-Cruise-Dinner-and-Schonbrunn-Palace-Concert/d454-3773VIEDONAU1, or one like it. It includes a cruise on the Danube IN Vienna and then the dinner and concert. If all three events are of interest then, if the price was reasonable, I'd say: sure it's worth it (as long as you know what the cruise is, and I'd read reviews of a tour like this carefully). But it's not the cruise on the Danube that sails past castles and villages and vineyards of the Wachau Valley.

If it was me, I'd use one day to do the 'train, Melk Abbey, Danube cruise, train' outing. I did this and it was so worth it. If you're interested in a concert and dinner you could do that separately on one of your other evenings.

All of these activities are tourist traps.

Tourist traps are the spice of life. Or one person's tourist trap is another person's excellent evening.

But you are correct. The OP should train down to Budapest where the views on the city Danube cruise are breath taking. But, again 90% tourist.