Did I really gain 3 lbs from one dinner out?

In my experience it takes 2-4 days for me to release any water retained from a high sodium meal.

It's likely water weight. Just up your water intake and give it a few days. It can take me a whole week to reverse the damage done by one night of eating out when it comes to sodium.

Did you really gain 3 pounds of fat from one meal? Certainly not, unless that was a 10,500 calorie steak.
Did you really gain 3 pounds of something? Yes, the scale says so.

Now, what is that something? Glycogen and water most likely.

On an aside, you're at the low end of a healthy weight range - and you need to realize that maintenance is always going to be a range, not a weight. You will always gain and lose small amounts as lean mass (mainly water) changes. That's simply living. You can't react to each time this happens with a calorie restriction, it's not healthy for body or mind.

Rather than a goal weight, set a goal range. For sake of argument, let's say that is 105-120 for you, which is thin without (quite) being underweight. Keep eating maintenance, and keep tracking weight. As long as you're in the range, do nothing more than that - don't adjust calories at all. Then you set thresholds where you take interventions - maybe above 120, you try to decrease your calories by 250 per day. If you were above 130, you'd reduce to a 500 per day deficit, etc. Likewise, if you fell below 105, you'd eat at a surplus - you have triggers on both ends.

That's one of the habits common to most of those who successfully diet and keep weight off. Monitor and set thresholds for intervention, but you can't react to every change, because at maintenance, your weight is still going to be changing all the time, even if your fat remains precisely the same.

Don't even worry about this. I woke up Saturday weighing 8 lbs more. But it's no reason to fret. Water retention, hormonal fluctuations and so on is normal. Stay off the scale for a while ( its taken around 5-6 days for me sometimes!) So just continue on with your normal calories , and drink your normal water intake and it will go away.

Water weight. You haven't gained fat from eating at maintenance, it's just not possible - Don't worry.

If you weigh yourself daily, or even occasionally more than once on a day, you will see fluctuations that would surprise you. Your trend might still be down (or maintaining) but the actual day to day weight can vary quite a bit. I've seen 5+ pound variations in a day, just depending on how hydrated I am.