Three years ago I purchased three grapevines and they are not producing grapes but instead are producing a funny looking prickly round object. Does anyone know what it could be?
Could be Wild Cucumber (single seedpods) or Bur Cucumber (cluster seedpods). Both are considered to be weeds.
What reputable nursery could make that horrible mistake?
Here is a link that might be useful: Wild Cucumber
It is possible that you do have grapes that require another variety for a pollination. Without pollination, the grape blossom will not develop, but may turn into a rudimentary stem with tiny stems having tiny bulb ends where the grapes would be.
Another possibility is that your vines are blooming too early for your season, and the blossoms are being killed by late frost/freeze.