A.T.
If you're north of Denver near me I can recommend Golden Bear Bike Shop in Broomfield. Lots of comfy commuter style bikes, and cruisers...not to mention cross-overs, mountain bikes, and roadies!
The newest child carrier I've seen is a WeeHoo, kind of like Trek's Tagalong (a half bike that sits behind parent bike), but the WeeHoo is better I think. I'm excited to try it out.
We currently have a cheap trailer, which has works okay, but hasn't held up under heavy use, and I know it's not super comfortable to sleep in---my daughter gets bored & tired back there.
The seats that sit behind are fine for a while, but soon enough your youngest's knees will be pushing on your behind, and he might be too heavy already to comfortably have him up front.
Let the teenagers pick their bikes, and I recommend the WeeHoo for the 3 yr old as it'll be great up until he's 6-ish.
Great job mommin' & welcome to Colorado!