I recommend UA fares rather than US fares.

As you pointed out, these are their hub cities and many US fares don't allow multiple connections.

From: CLT:: US US US
To: BOS:: US US US

This syntax works IF you have known routing rule allows double connections. Yes, ITA may price it out at $291, but by using two different fare components, which usually increases total price.

e.g. CLT-IAD-LGA-BOS and v.v. is available (including US metal) at $186.10 all-in in March