Since your daughter is only 3 years old, she really doesn't need to know her letters and numbers just yet. If she just happens to learn them, along the way, while doing fun activities, I think that's okay. But doing flashcards at her age might be stressing her out.
The best advice I could ever give you is to read, read, read books to her. Read to her all throughout the day. Read as long as she will listen and as long as she is enjoying it. If the book is an alphabet book or a number book, then she will learn her letters and numbers but she won't feel like she's being drilled. She will just think she's having fun and won't even realize that she's learning.
When you read books to her, you will be instilling a love of reading and a love of learning in her. It's also a good idea to take her at least once a week or so to your local public library. Allow her to check out books that interest her.
She is also old enough to be listening to books that have cassette tapes or a cd with them. She can listen and look at the pictures in the book. This is a valuable experience. My 2 year old daughter listens to regular old fashioned records that have a book with them. (They were mine when I was a child.) She likes to listen to them and look at the book while she eats breakfast and lunch. (You can check out the books w/cds at the library.)
A really fun site that your daughter would enjoy is starfall.
And here is a link about homeschooling 3 and 4 yr. olds that might be helpful.