Yes weight trends to increase throughout the day. Do yourself a favor and don’t weight yourself several times a day. Weigh yourself once 1st thing every morning after peeing, & compare this mornings weight to the same mornings weight next week.
It is very possible — even easy — to drink three pounds of water over the day. There’s an old saying, “A pint’s a pound the world around.” It means one pint of water (16 fl oz) weighs about a pound. So if you drank three pint glasses’ worth of water over the day, then yes, you drank three pounds of water. That really isn’t hard to do! In fact, you really should be drinking at least that much water so you don’t get dehydrated.