what if it is 3 volt and 3.5 volt LEDs?

A 3V LED will drop 3V across it and requires a voltage not only higher than it but a voltage higher than the current limiting resistance that has to be in series with the LED and the voltage source. This is because there is no current limit on a battery (other than it's internal resistance which is quite small !)

So the equation would be V = I*R + Vd; where Vd is the diode voltage drop.

The equation is highly dependant on the amount of current thru the resistor, as the current restriction will determine the voltage drop of the resistor in addition to the LED for the minimum source (battery) voltage.

e.g. a blue LED with a 3V drop and 20mA (0.020A) of current will need a 15 ohm resistor for a 3.3V battery since 3.3V = (0.020)*(15) + 3V = 0.3V + 3V

The wattage for the resistor is I^2 * R = (0.020)^2 * 15 = 6 milliWatts so any resistor wattage (1W, 1/2W, 1/4W, 1/8W, 1/10W, 1/16W, 1/32W) will do.

A 3.5 volt LED will act in a similiar manner and require an even higher voltage, with a minimum of 0.5V.

its just consuming more power.