Don’t know of any clam-shells (where the zipper divided the bag into 2 equal sections) than are less than 3lb. If panel loading is ok, then around 2lb are travel packs from ula-equipment, the now discontinued osprey ozone 46l, and if a duffel is ok, there is a 2lb5oz tnf 42l basecamp voyager duffel. The Mountainsmith Wraith from the early 2000s was a very nice panel loader than weighted less than 2lb, but hasn’t been sold for year :(. There are countless travel packs which show up on kickstarter… you might be able to find some project there that is now successfully shipping that fits your spec. I used to maintain a list of travel packs but stopped a few years ago so the list is going stall.
Personally, I don’t pay as much attention to weight for urban travel because I am carrying my bag shorter distance than in the back country. When I mix back country with travel I use my 40L Gossamer Gear Gorilla which is far from ideal for urban, but works in a pinch.I was sort of hoping the Hyperlite Mountain Gear SouthWest 2400 might become my do-everything bag, but it just didn’t work for me.
For urban travel I generally use a Tom Bihn Synik 30L which isn’t light but works well for me. I have done a few trips with the packable version of the gossamer gear vagabond which worked surprisingly well when I could get away with a single laptop (normally need more computing devices for work trips) and didn’t have to bring a suit.