Do the LEDs really have a 3.3V forward voltage? In that case you are limited to one per pin. I think some external circuitry is starting to look good here.
Having a look at the data sheet is always good.
I just picked any Kingbright UV LED ( rated VF typ 3.8 max 4.2 V @20mA) , which seems to support Nick's Statement.
I'm only right with my assumption about the resulting current to be less than 20 mA 
According to the attached diagram it should be around 0.0x mA @2.5V (which would not harm the Arduino), and the voltage drop would typically vary only by 0.7V with a current variation of 5 to 25 mA.
I fully agree with Nick that "external circuitry is starting to look good here". It increases the possible scenarios a lot.
My approach however is rather experimenting with what you have got already, and experience the encountered limits.
