If you don't want two day trips, then they should be frozen lower (sooner) in your bid group..

Something like this :
Set "something"
Set "something else"
Award work L--
Avoid trip <3 days
Award work L-
Avoid trip depart from BWI
Award work L
Avoid trip depart DCA
Award work N


This will freeze all the two day trips at the lowest level. When you click on the sort at the bottom of that bid group, you should see all the two day trips sorted into L--, then all the 3 and 4 day trips out of BWI in the L- bucket, all the 3 / 4 day DCA trips in the L bucket, and all the 3 / 4 day IAD trips in the Neutral bucket. The sequence of your award and avoid statements is significant and will impact what bucket the trips are in.