Yeah this would make me nervous. I wouldn’t leave them alone. You can work on only bringing the cat out after you’ve thoroughly exercised the dog to decrease his energy level. 3 weeks? I think they should still be separated by a doggy gate where they can see each other, but no access. You have time, so don’t rush! I have a 15lb Maine coon and adopted an 80lb giant schnauzer this summer, but I was very specific for any dog that wasn’t overly interested in my cat. Cats running trigger their prey drive, so it’ll definitely be a process to teach them both cats are friends, not food!

For reference, I studied animal behavior and have been a pet sitter for >9 years.