I think it would be TMI for a 3 y/o. All gender/sex/erection things/questions have to be treated matter-of-factly and with only the amount of info child seeking (the key is NOT more!)
I have a 3.5 y/o and he noticed that girls, and me do not have what he have.
This conversation went like that:
-mommy, you do not have a pi-pi...
-no I do not.
-your pi-pi comes from your butt?
-no, I have a little hole for that... all girls have...boys have big pi-pi and girls have very small.
-is it painfull?
-not at all, it feels the same as for you.
-OK
-you have any more questions?
-no.
And that was it! No more details. I have a now 12 y older boy and he had a question at 5 "Where the babies come from?". She became very confused and sent him to me. That conversation went like that:
-Mommy, where the babies come from?
-Dear, what is it you want to know?
-I want to know if the baby really lives in the belly and then comes out?
-That is exactly how it happens.
-But how does it come out?
-Mommy has a special small opening called vagina, when the baby is ready mommy stretches a lot down there and the baby is able to come out.
-Does it hurt?
-Yes, it does, sometimes very much.
-Did it hurt when I came out?
-Yes it did.
-But you are OK now?
-I am all good now and very glad I have you. Is there anything else you wanted to ask?
-No, just that. Thank you.
And through the years I had to answer questions like "What is gay?", "Who is a pedofile?", "What is wet dream? Will I have one?" You get the picture.
I follow the same script always:
1. Find out what the child already knows.
2. Answer the specific question, no more no less.
3. Ask if there is more the child wants to know.
Never push that sort of info on the kid - all will come in due time.
Hope that helps. Good luck.