Check the experience amount given per creature, multiply by the number of creatures, and compare back to the experience per CR chart. In this case it's 135 XP per CR 1/3 Drow. Three of them combined is just a little more than 400 XP (it's 405 XP, which is probably due to them not wanting to use 133 XP each), so it's a CR 1 fight.
It would be approximately a CR 1. Look at the XP totals. 3 CR 1/3 creatures is 405 xp. A CR 1 encounter is 400 xp. If you want to throw a CR 3 encounter at the party, it should add up to 800 xp worth of monsters. a CR 1/3 is 135 xp, so you would need about 6 of them (its technically 810 xp, but that is close enough).
That is the best way to look at it. The table for multiple monsters = CR + # gets screwy when dealing with CR < 1 in my experience.
