sounds like its in the normal range. all kids are different, and your son being so NOT like that, probaly makes her seem MORE like that, kwim. if you only had her, you might not even think anything of it.
i do though think starting preschool now is a good thing. some kids take longer to get used to it.
that all being said, i think time out at 3 is fine, but i think it needs to be done the proper way(serious misbehaving only, warning first, minute for age, talk about it after- sorry, love you). in your case, i wouldnt give a time out for tantrums, but would if she is speaking to you in a disrespectful way during it. for tantrums though, i would ask her to go to her room until she calms down. i would just explain to her i cant understand her when she is talking like that and she is hurting my ears. at 3 1/2, my daughter was able to comprehend that. often, when she was calm, she didnt even mention the thing that first set her off. i dont like to make the time out offenses to be the same as tantrums so i try to separate behavious.
aside from all that, i totally agree with the other poster about choices. at first, she might not agree to it, but if you are consistent, she will look forward to hear the choices and pick.