Spray the crock of a 6 quart oval crockpot or similar with cooking spray.
Pour 2 cans of peaches with syrup, and peaches only from the 3rd can, into the bottom of the slow cooker. Add vanilla and cinnamon.
Dissolve cornstarch in just enough of reserved syrup (from 3rd can) to make it smooth. Pour into crock, gently stir to combine.
In a medium-size mixing bowl, whisk together butter, milk and egg. Add cake mix fluffing with a fork until fully moistened. Dollop evenly over peaches. They don't have to be completely covered. Leave a ½ inch border along the outer edge to allow for expansion.
Place doubled paper towels over opening of slow cooker, then place lid firmly on top.
Bake on high for 2 ½ - 3 hours. Uncover and check center of cake for doneness at 2 hours with a toothpick or cake tester looking for a clean toothpick. If it needs additional time, replace paper towel and cover.
Once done, let stand uncovered for 10-15 minutes for syrup to thicken, then serve with vanilla ice cream or whipped cream.