First, drain most (maybe 80%) of the juice from the pineapple, peaches, and cherries. Dump all three cans in a Crock Pot slow cooker.
Sprinkle corn starch and sugar over the fruit. Mix in until there are no lumps.
Pour the cake mix over the fruit. (Don't stir.) Pour the oatmeal over the cake mix and lightly mix it in to the cake mix with your fingers.
Slice the butter into 1/2" pats and place evenly over the top of the cake mix. (This is where the "trust the process" part of this recipe comes in...it works, I promise!)
Set your slow cooker to low, 4 hours. Cake will be finished when ends are cooked and fruit filling is slightly bubbling out.
Serve with vanilla ice cream or whipped cream.