Hi Sully and welcome to the forum,
If you open up those spaces, make sure you build insulated doors and air seal them well. When my cape was built, they put a shed dormer on the back to extend that side of two bedrooms and make space for the bathroom. Works well, but the flatter roof has snow and ice issues I have to deal with. My side walls are about 4' so the space is easy to get into. At 3' you will have limited access. I have worked on capes where they finished those slanted ceilings all the way down to the eaves, no walls. Then used built-ins in front of that wedge space for storage. Not a difficult task, but would want to super insulate and be sure the rafters remained vented.
Here is a link about capes that might help if you do any construction:
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Bud