Three Cups Walk is a short path leading from Ely Market Square to Forehill car-park. The footpath can offer a nice view through the trees to the east side of Ely Cathedral. If anyone knows the reason behind the name for this path, we'd love to know!
Edit - it turns out the building at the top of the path on Forehill used to be the three cups pub. That explains it!
You're looking for a small clip-lock container, big enough to hold small swaps and TBs.
Take care hunting- muggles can approach from two different directions, so some stealth is necessary, other than that it should be an easy find though!