Center Glow is a 6-8 foot high and wide ninebark, and I don't think that ninebarks look great when pruned for width, so I wouldn't put that in a 3' wide spot. (I think even the smallest ninebarks are unlikely to fit there.)

In my experience, PJM rhododendrons tend to be about 6 feet in all directions, so it would take quite a bit of pruning to keep them out of the walkway.

I am not familiar with Grayswood hydrangea, but I do know that there are several of the newer varieties of hydrangea that are supposed to stay at around 3-4 feet in all directions, so a small hydrangea may be your best bet, or you can try finding a shrub that has been selected for narrow growth. IME, it's a lot of work to keep a shrub that wants to grow large looking well-shaped and blooming at a smaller size.

I have a Grayswood hydrangea that hasn't exceeded 3x3' in 4 years. It also grows upright and doesn't splay out.

My Grayswood is 5 and a half feet tall and maybe 2-3 wide as it grows upright for me and is not a mounding plant. One should work fine.