Rapid Weight Gain

I agree that most of it is water retention and food in the body-pipes on its way to the toilet. For 3 pounds to be fat, you'd have to have eaten roughly 10,500 calories above your current weight-maintenance calories. While that's theoretically possible to do, it isn't super likely.

You had a lot of carbs and sodium/salt in there. Those increase water retention temporarily.

Add up the estimated calories you ate all day. (You did log it, didn't you? If not, do it now. Estimate as necessary.) Compare that total to your current estimated weight-maintenance calories (not just your weight loss calorie goal). The number of calories over maintenance calories is the estimated potential maximum fat gain for that day.

Example, using round numbers for convenience: Let's say my weight-maintenance calories are 2000, and my MFP calorie goal is 1500. One day I go wild, and eat literally double my maintenance calories: 4000 calories total intake.

Yikes? Nah, not really. It would be 2000 calories of extra body fat, at the theoretical extreme. That's about 0.57 pounds. Since my usual goal in the example was 1500 (for a pound a week weight loss), I wiped out loss that day, and three subsequent days, at most. (2000 over, divided by 500 daily deficit, 4 days delay including the day of. At most.)

When I was losing, I'd do rough estimates like that to decide how long I'd delay getting to goal weight if I over-ate in a certain rare case (examples: birthday, Christmas dinner). Sometimes that delay was worth it to me, and I ate the extra food. Sometimes it wasn't. I either didn't eat it, or rethought my plans after doing it to avoid repeats.

Note that the maximum fat gain based on calories probably won't happen. I'm not going to go into details (could, but won't) about why. In brief, it's that human bodies are dynamic: Calorie intake affects calorie expenditure. It's from things like over-eating (compared to normal intake) tending to juice our energy level, including unconscious things like interior body temperature and heart rate. We tend to burn a few more calories after this kind of scenario. That slightly limits the fat gain, but it's for sure not a "get out of jail free" pass.

I've actually over-eaten multiple times while losing, and many times in the roughly 8 years of maintenance since. The impact is usually pretty minor, at most. I wrote up one personal "case study" in a thread, here (and there are links in there to other similar threads from other people):

https://community.myfitnesspal.com/en/discussion/10603949/big-overfeed-ruins-everything-nope#latest

I'd suggest you do two things:

1. Go back to your regular healthy routine. Don't try to make up for it in any way, just go back to your calorie goal.

2. If it wasn't worth it to you - sounds like it wasn't - think about why it happened, and spend no more than 10 minutes doing that plus revising your weight loss tactics to avoid a recurrence. (Examples: Did you let yourself get over-hungry? Did you go into a tempting restaurant scenario without a firm plan for moderation? Was there an emotional component, high stress, or sleep deficit, or something like that, where you should deal with the root cause rather than turning to food? Did you over-exercise? Are you being too restrictive in your current plans, either too few calories or putting tasty foods you could moderate totally off limits?) Figure out the triggers/causes, revise your plan to avoid repeats, rehearse the new plan vividly in your head a few times (like a mini movie) so it kicks in next time you're in a similar situation.

Then just get back on plan. In something from a couple of days up to a couple of weeks, the water weight/food waste will be out of the picture, and you'll have a clearer idea of the actual impact.

Best wishes!

If you’ve been losing then you won’t gain you’ll just lose more slowly for a week or 2 or not lose depending on you recent rate of loss.

Some people that have been on low calories actually get a whoosh of water loss a few days after having a higher calorie day.

You have temporarily water weight gain from salt and carbs so don’t worry about it.