Why isn't my rabbit drinking any water?

She is probably getting the water from the cucumbers.
Different rabbits will also drink different amounts of water.

People say to introduce veggies at 3 months old. But i used to breed bunnies and i always gave them veggies when they were little and they never got sick. Im wondering how my bunnies were fine while every other persons bunny couldnt have any veggies.

No. Stop the cucumbers immediately! This can kill her at this age by causing digestive upset or diarrhea. She is too young. Greens and veggies shouldn't be introduced for another 4 weeks (when she is 12 weeks of age) at the earliest.

Here are the feeding recommendations from the House Rabbit Society so you can see:
• 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.)

The fact that she is eating hay is excellent. That is actually better than eating pellets and no hay. If you switched the pellets, that could be why she refuses them. They don't take well to sudden changes in pellet type. The store should have given you some of the same type and brand of pellets that she was used to eating. Then these pellets they give you should be used to transition her slowly to whatever healthy brand you have chosen to use.

You can take a look at my site to see how to choose a healthy pellet. There is also further information on feeding. Just bear in mind that rabbits under 6 months of age have a different diet than adult rabbits. It sounds like you are doing well already by providing alfalfa hay (good for her age) and water bowl. Just avoid any greens, cucumbers or veggies until she is older.

http://rabbitsindoors.weebly.com/feeding.html

She gets the water she need from the cucumbers. They're almost all water. Bunnies don't need a lot of water, especially if she's not eating the pellets. If she's healthy, she knows when and when not to drink. She's a cutie!