I would definitely not worry about 2-3lb fluctuations. I use the scale for one primary purpose: weighing myself before the ride, and weighing myself after, to see how much water weight I lost, so I know how much to put back in. I took in six 25oz bottles in a 4.5 hour ride (over 8 pounds of water) and still ended the ride 5lbs lighter than when I started.

Body weight can only be looked at over time, to track gains and losses over extended periods of time. I lost 50lbs on the bike, but will still fluctuate 7-12lbs on any given ride day. Also, that whole "muscle is heavier than fat" adage does apply. My waistband went down 5", and even though I've put on about 3lbs over the past 6 weeks, my clothes are still getting looser.

Obviously, everyone is different-- but I remain horrifically inefficient, burning over 700 calories per hour on low-Z2 efforts. Any "spirited" ride is 900-1000/hr, and all-out efforts can go 1200-1300 per hour. And yes, this is with a power meter. For instance, this morning was a mixed Z2/Z3 effort, where I burned ~55kcal/mi, just a bit over 1,000kcal/hr. But it was cool this morning, so I'll likely end the day a few pounds heavier than I started it, by remaining nice and hydrated.