My DD was pretty small (6 lbs 1 oz) and the ONLY cover that fit for the first couple weeks was the Bummies XS (6-8 lbs). I only had one though--Thirsties duos size 1 and XS Thirsties worked pretty well, and were really nice after that for the whole XS prefold diaper size time period (till 10 lbs, I think).
Mom to the wacky and wonderful Kalyani (August 2011)
For newborn covers, I would also recommend the Bummis covers in the smallest size. The Super Brights go smaller than the Super Whisper Wraps. The main other difference between the two is that the Super Brights have a wipe-able inside and the SWW have cloth inside. These worked for our son until he was much bigger than the posted weight range, and he was big around the waist, so they were a good value for us.
Amy - married to my best friend and mom to DS born September, 2011.
We had size small covers which worked okay-ish, but I didn't find the 3 cover scheme realistic because they got dirty when she pooped and she pooped a lot.
I like Bummies wisper wraps and Super Brites (Their XS is premie tiny almost, average 7 lber is great in the small) I also love their new pull on.
Thirsties never wear as well for me with their thinner PUL.
Pro Wraps Newborn size are good too, but no laundry tabs and very cheap.
I would plan on more than three since they will get pooped on lots (4 would be nicer, two in the wash/drying, one on, one to air/spare)
I would also make a few more flats (I love receiving blanket flats), for 18 total, in case you have a real pooper, want to stretch washing a little, or like me Bee, have a newborn who does not poop at night and hates having her diaper changed when she wants to go back to sleep, and need to double them up.
Mama to Monkey (Jan '09), Bee (May '11), and Cat (August, '13)
I highly recommend the Thirsties Duo Wraps. They have a double leg gusset which prevented a lot of leg-leakage for us; plus, they have an adjustable rise so they fit well from newborn until about 5 months (for me, anyway- and my baby was very average in size, born at 7 lbs 10 oz).