Although winning the championship will be tough, they have done it three times.

Although winning the championship is tough, they've managed to do it on three seperate occasions. Hope this helps :)

The compound sentence has good structure as is. It does not need to be improved. Here is an alternate way to say the same thing: "To win a championship is tough, but they have beat the odds and done three times." I prefer "to win" to "winning" because infinitives make starker statements than participles.

Your structure is grammatically sound and reasonable. No need to change anything.. An alternative: Although winning a championship is tough, they've managed it three times. "managed it" means that they accomplished it and implies further that it was a difficult endeavor.

There's nothing wrong with your structure. It's simple but clear. You could add words to embellish it, and to soften it, especially for well educated native speakers, but then it would be less clear for people with poor English, and less respected by people who value simplicity for accessibility. If you can provide the surrounding sentences, then maybe we can improve this sentence to fit within them, but as it stands, as a standalone sentence, I wouldn't change anything.