So I have to limit the power to 10.5~12V
You mean limit the voltage not the power and no you do not have to limit it.
Make sure the LEDs are rated for 3.5V forward voltage drop because that sounds high to me. The resistor limits the current going through the LEDs (same current goes through each if they are in series), this current must be below the specification of the LED you use, 20mA is normal but as have been stated 10mA is usually fine. When calculating the resistor value always take the lower voltage readings from the specification.
An LED is what is known as a non liner device, that is the relationship between current and voltage is not a straight line if plotted on a graph. That means you can't just give it a voltage and let it take care of itself, you have to control the current. A simply way of controlling the current is through a series resistor, but make your calculations based on the maximum input voltage it will experience.