Use the main lot and walk.

There is a tiny parking lot on Goat Island near Three Sisters Islands, but it has maybe 20 spots and fills up fast on any decent day. You’ll waste more time circling than you would just walking from the main lot.

The walk from the main Niagara Falls State Park lot across the bridge to Goat Island and out to Three Sisters is maybe 10–15 minutes. Easy. Scenic. You pass the falls. It’s part of the experience.

Unless you arrive at 7 AM on a Tuesday in February, plan on the main lot. You’ll save yourself the headache.