Part of your challenge is that this very narrow bed is adjacent to a walkway. That means that the plants you choose cannot grow to be in the way of foot traffic.

You'll install the plants at the midway line of that three foot area, leaving very little leeway for overhanging branches. You will find it aggravating if that walkway becomes narrower as the plant grows, and you may resort to pruning shrubs into little unhappy meatballs. /:-(

Tell us your general location, and I'll bet we can come up with some good ideas for you to think about.

That’s a very good point and definitely something to consider. I’m located in Ottawa ON.

You're on the right track with the hostas. If you have afternoon shade you might want to consider a small hydrangea called "Bobo". If you are looking to the corners you could consider "Little Lime" as its a bit taller. Also rhododendrons are wonderful in afternoon shade, but you'd have a hard time keeping them small enough unless you could find a short enough variety or put them on the corners where you have more space for the height and width. Rhododendrons will give you leaves year round as they don't drop them in the fall. You might want to add some small evergreens for year round appeal too. There are not many that will grow narrow and thin (on either side of the window, or corners), but you might look into that. If you'd like a small rounded evergreen you can't beat "Danica" cedar.

For more vertical interest you could also add trellis or supports on either side of that window and grow clematis (check which varieties will take less sun) or other vines tolerant to afternoon shade.

Thank you @mary_rockland - this is all great advice! Perfect timing too since it looks like spring may finally be arriving in my area!