In the app, just select a car with a car seat. As soon as the driver accepts u to pick you up, you are then able to contact them. I suggest calling and letting them know you need 3 car seats. Also, you might want to try selecting the "SUV With car seat" option and then do the same thing (contact them and ask if they have 3 car seats). Drivers with a larger car like an suv are the most likely candidate to have more than one car seat. But at this time, as far as i know, there is no way to request more than one car seat specifically. I would actually suggest that you email uber directly and ask them this question. Then they will know that this is a good idea to add as an option in their app (to be able to request ubers with a certain # of car seats). They are pretty good at responding to emails promptly.