My Fothergilla are beautiful and get better and better every year, but I can't grow Clethra successfully worth a darn, more than I care to admit have met their demise in my yard, which leads me to believe cultural requirements of these two shrubs differ to some degree. Or maybe I'm just the resident Clethra-killer, not really sure...

Clethra is really, really slow to leaf out, even slower than itea. Mine still haven't leafed out yet, but they do have teeny, tiny buds and green under the stems. (I check mine for signs of life every year, lol). The dried seedheads still look cool, and their fragrance and color make it worth the wait. So I tell myself...

Knowing that you won't have much to look at, shrub-wise, until later, I would use an evergreen like a pulmonaria or an autumn fern (dryopteris), to focus attention on the front of the bed. Autumn ferns look really nice with a blue hosta like Halcyon, their new foliage is orange. Or, if you have moist soil, brunnera in front of woodland fern (athyrium) is a sure bet. I would still use hardy geraniums for summer long color, lady's mantle (alchemilla) for a pop of chartreuse, and anemone, ligularia, aruncus or actaea for later fall color.

The lady's mantle might be another good consideration.

The good news with the late leafing clethra is that the entire bed is lined with spring bulbs.

I temporaily put a few boxwoods in this area (storage before I plant them) and they look really nice there...just a little too big. I have room for (3) 2' X 2' plants...or larger number of smaller plants.

You've gotten some great suggestions so far, but I'd like to also suggest a blue oats grass for texture and color, or festucas if you want something smaller. These would give you color year round.

Hmmm...lady's mantle with the blue oat grass might be quite striking.

Or a blue oak grass with a Karmina geranium...might not get much fall interest with that combo though.

But the clethra and fothergila should pick up the slack at that point.

If you only have room for three 2' x 2' plants, you'll get the most impact from all three plants being the same. Last year, I would've swore by Rozanne geraniums, but now that I've seen pulmonaria Mrs. Moon thriving in a clump along the edge of my woodland-ish garden, it's a toss up. The pulmonaria is nearly evergreen, the foliage is always interesting, and the flowers change color from pink to blue! The leaves on this one are short, so they don't get as ratty in the winter as others I've tried. They spread nicely too, will fill in quickly. Good for covering bulbs!