Not knowing your surgeon's protocols, all I can give you is my personal experience.
I registered for surgery at 244 lbs.
It took me a very, very long time to jump through all my insurance's hoops and get approval. Almost a year. By the time my pre-op orientation rolled around, three weeks before my surgery, I was told that they expected me to have lost at least 5 lbs since my registration. "No problem", I thought. Just to be helpful, when they gave us this information at the orientation, they offered to weigh everyone before we left and compare it to our registraion weight so we could see where we were. I expected to be well below my initial 244, but alas--I had actually gained 7 lbs.
I had spent a lot of time during the very lengthy approval process having "last meals" and saying goodbye to all my favorite things. And, like you, I had quit smoking. I guess it really snuck up on me.
So in the following three weeks, I had to lose not only the 7lbs I gained, but a additional 5 to meet my surgeon's expectations. I don't know if he would have actually postponed me or not if I didn't meet that goal. Probably not. But I wasn't willing to test it.
I was given no specific pre-op diet. He told me to stick to less than 1300 calories a day, no soda, high protein, low carb. I went a little extreme and whittled my calories down to 700-800 a day. I ended up losing 11lbs and had my surgery done at 233.
So, is it possible? Absolutely. Of course, I had three weeks. You have three days. Do you know for sure they will be weighing you at your pre-op class?
If they weight you at your pre-op class, you could explain your recent salt intake and ask that they reschedule you another pre-op class on the day before your surgery to reweigh you. This would give you some extra time.
Water retention can be fixed relatively quickly--stay away from salt, alcohol, and caffeine. With the heat everyone's been getting, go take some long walks and sweat it out. You can do this. Three days isn't much, and you likely won't be able to dodge the entire 8 lbs, but if it's water weight, you could shift a big portion of it.
Good luck, you can do it!
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Check out my video blog! www.youtube.com/user/HappilyShrinking/videos
Highest weight: 269. Surgery weight: 233. Goal weight: 144, and then we'll see..