Calves that young need a bottle twice a day. Drinking from a bucket does not place the milk/replacer in the correct area for the calves to digest. They must be bottle fed. The sucking action plus having the neck in position as nursing (with a curve in the neck and the nose up), also allows the milk to be placed in the correct stomach for a calf. What are you putting in the bucket? They need milk replacer. One bottle in the morning and one at night.
AFter you get that going for you, then you can leave out fresh water in a bucket, and a little sample of sweet feed or small calf plellets. They will need to be bottle fed for at least 6 weeks.
Definitely get with a good cattle vet.
Good luck. Orphan calves are fragile and risky.