I call this a "triple trillium" due to the multi flower petals. However, I think the proper name may be Double Flowered T. Grandiflorum. I have been 'nursing' this for nearly fifteen years, starting with three stems and flowers. This year it has nine stems and flowers. It began life near Orillia on the edge of the hayfield on our property. When I discovered it, a couple of years before we moved, I was determined to take it with me. It was amongst hundreds of 'normal' white trilliums.
Up here, in N.Ontario, there are no white trilliums, and I'm so happy this has survived. We have many single deep red trilliums behind the house in the forest which really 'show off' this white one. It's just sooo pretty.