In a saucepan, combine maple syrup, cinnamon, and apples. Simmer until apples are tender. Divide the apples and place equally into 6 buttered ramekins.
In a mixing bowl, whisk egg (yolks) and the 3 eggs until combined. Add milk, sugar, and vanilla. Pour mixture into ramekins and cover with aluminum foil.
Place steamer rack in bottom of cooker. Pour in water. Place ramekins into cooker. Place as many as will fit, but do not overcrowd.
Close and lock lid and set pressure cooker to "bake", adjust time to 5 minutes, and press "start/stop". When done, natural release for 10 minutes, then do a quick release to make sure all pressure is released before opening lid.
Carefully open lid and carefully remove ramekins. Repeat, if necessary, with any remaining ramekins. Place ramekins into fridge to cool flan before serving.