Yes, you may request the safety car seat for inflight use. But you need to book it in advance. I personally don't recommend it for 2 reasons:
1. For a 3 years child, you should be using a proper seat belt fitted on the seat.
2. For comfort, it is not comfortable. It is nothing like a car seat you see normally. It's a heavy piece of car seat looking equipment design for aircraft use.
3. Lastly, this seat is actually design for an adult passenger travelling with 2 children under 2 years of age. As children under 2 years old are meant to be held by an adult with a child restraint device (child seat belt) for take off and landing. Each adult passenger can only hold on to 1 child, so this equipment would allow 1 adult to travel with 2 very young children.