@willay congratulations and you have a tough choice here. I don’t have a connection to any of the three schools, so I will try to give you neutral opinions.
I would start by saying be careful of older stereotypes that adults have. They may have been born in truth, but have slowly melted away. So that would apply to Brown here. That said, if you graduate from Cornell with a degree in CS, people know you are tough and are not a lightweight.
I think I drop Cornell in part because of the weather. I think it is an awesome school, but given your other choices you can choose something that is more comfortable. If you were hardcore CS only, it would probably be the best choice, since they are amazing in CS.
That leaves Duke and Brown. Duke is solid and wins weather of course. I don’t think Duke is a bad choice and would serve your needs, especially with the double major. There’s a lot of great stuff going on there in terms of research and student programs. The sports culture is there and you would probably really enjoy it.
But I like Brown for you the best. As long as you put in the hard work, it will show at ANY of the 3 schools. Your multiple interests is a perfect fit for their open curriculum. Their strength in CS and especially Applied Math (which is particularly multidisciplinary) stands out. If you have high level classes in CS, Applied Math, and Ec there with good grades, you won’t be thought of as a “lightweight.” And when I saw it, it was a fairly chill, happy place. No screaming at people for being moderate. And all these places have niches where you would be fine.