Ok, I know it's been about a year since I've seen vine growth, but, is Frontenac really supposed to have three clusters? I don't remember thinning last year. Why does my vine have so many clusters?
Frontenac normally has 2 or 3 but sometimes 4 clusters. Just wait until they are a manageble size and pinch the extras off with your nails. Generally the first and second are the largest so just pinch off the others. Alternately you can prune out some of the shoots so you end up with the right number of clusters but if you thin the actual clusters they generally get larger.