First drawing forward drop of LED = 3v ( you told us so )
second drawing, no current as you have 2 3v LEDs in series
third drawing - damage, 2 3v LEDs will try to drag the output of the Arduino down to 3v
third drawing, 5v less the 3 volts accross the diode = 2v accross the 250R res, by ohms law = 8 Ma each LED