Among the three brands you mentioned, Harley-Davidson has traditionally offered the largest engine sizes. This engine is available in select Harley-Davidson models, such as the CVO (Custom Vehicle Operations) lineup.
As for Ducati, they are known for their high-performance motorcycles, but their engine sizes typically range between 800 cc to 1,300 cc, with the largest engine being the Ducati 1299 Panigale, which has a displacement of 1,285 cc.
Triumph, on the other hand, offers a range of engine sizes, with their largest production engine being the Triumph Rocket 3, which has a displacement of 2,458 cc. The Rocket 3 models are renowned for their massive three-cylinder engines.
It's worth noting that motorcycle manufacturers may introduce new models and engine sizes over time, so it's always a good idea to consult the latest information from the respective manufacturers or dealerships to get the most up-to-date information on their current offerings.