Add the rice to the Instant Pot along with water, 2 cups of milk and sugar. Mix well and lock the lid. Make sure that the steam valve is closed.
Press Porridge button - this will automatically set the IP for 20 minutes on High Pressure. It will take about 10 minutes to come to pressure.
Once the pressure cooker beeps, let the pressure release naturally for 10 minutes. Then quick release the remainder.
In a small bowl, combine corn starch with the remaining 1 cup of milk and rosewater. Whisk well to make a slurry without any lumps. Add this to the Instant pot and mix well to combine.
Select 'Sauté' and cook, stirring continuously, until the mixture is creamy about 2~3 minutes.
At this point, carefully remove the steel pot from the housing. Leaving it on the heating element will cause the bottom to scorch.
Let the mixture cool slightly and divide it between serving bowls. Cover the top with plastic wrap and chill at least 2~3 hours before serving. Sprinkle with chopped pistachios and dried rose petals and serve.