At three, your child can do both without confusion. You identify them and she'll pick up on it. You say something like, "This is the number 1." "Here is the (upper/lowercase) letter A/a." You show her when you find the letters or numbers you are working on, or print the letter/number. She might even like to try printing or tracing the number. We played "I Spy" when we could. "I spy with my little eye -- a number (a letter) can you tell me which one it is?" If there were more than one letter or number, that helped the game feel like a game. Soon, our daughter was saying it back and we were looking for letters or numbers, too.
So, it is not necessary to start with letters or numbers first, but it is a good idea to say what they are. "Here is a number three/ or here is a letter B."
Sing the Alphabet Song and perhaps "This Old Man" for numbers. Say, "This is the Alphabet Song. It is about all of the letters." LINK to song "This is the song about counting Numbers. LINK to song
There are other number and alphabet songs. Music really helps make learning fun and easier to remember. (I still sing ABC to myself if I need to know letter placement.)