Call your vet!!! That is a serious medical question. Of course, after 3 days, God just might be shining on you...
I would definitely have him checked out by the vet. Rattlesnakes are the snake that is most likely to cause fatalities in dogs. A dog can usually survive a copperhead or water moccasin bite with no medical assistance but rattlesnake bites are different.
Where was the dog bitten? Is there a lot of swelling? With snakebites, it's not all that important to clean the area, which is good because the area of a bite is usually extremely painful to the dog and it can cause even a sweet-tempered dog who would never even think of growling to just up and bite.
After three days if the symptoms are not continuing to get worse I would say that things are probably going to work out, HOWEVER, the venom can be doing damage INSIDE the dog where you can't see it so the dog needs to be examined by a vet to make sure all is well and he is healing the way he's supposed to. After he gets over this there IS a vaccine for rattlesnake bites, so since you have them in your area it would be a good idea to have him vaccinated so he'll be protected in the event he runs into another one in the future.
Please let us know what the vet says and keep us updated on his condition.
Ghilly