According to the CDC, children should be in a rear or forward facing car seat until they are 40lbs. Check out the CDC website http://www.cdc.gov/MotorVehicleSafety/Child_Passenger_Safety/CPS-Factsheet.html
It is both, if you are using the back it is 30-100 if you remove the back your child has to be 40lbs so that is why there are the two different weights depends on how you use the seat. But each state is different where I live the law is to use a "booster" seat the child has to be 40lbs. In this case you can buy a backless. But in everything I have read it is safer to leave them with a back longer. Hope that helps!
Absolutely not. A three year old should be in a rear or forward facing harnessed seat, not a belt positioning booster. Please check out this link. http://csftl.org/harness-or-booster-when-to-make-the-switch/