Watermelons brings up an important point. When you go to get your rabbit, be sure to ask what your bunny has been eating and whether or not momma bunny was getting greens during and immediately after pregnancy. That way you'll know whether your bunny is ready for greens sooner.
The reason people recommend waiting until 12 weeks (3 months) is to give bunny's tummy time IF bunny is not accustomed to greens. Some breeders don't offer any greens (or very little) in which case bunny's tummy can react badly to greens if offered too soon.
Just go slowly when introducing greens -- one type at a time and looking for reaction (change in poos).
The House Rabbit Society has the following recommendations. These are based on the assumption that new baby had not been getting greens with momma.
• Birth to 3 weeks–mother’s milk
• 3 to 4 weeks–mother’s milk, nibbles of alfalfa and pellets
• 4 to 7 weeks–mother’s milk, access to alfalfa and pellets
• 7 weeks to 7 months–unlimited pellets, unlimited hay (plus see 12 weeks below)
• 12 weeks–introduce vegetables (one at a time, quantities under 1/2 oz.)
Some bunnies go crazy eating way too many pellets when offered them freely. If eating too many pellets causes them to eat less hay, that is the indication to reduce pellets enough to encourage plenty of hay eating.