For a 3 year old, the important things are to make sure she knows she can come to you any time she feels uncomfortable. Also, there's no need to go into detail about it being a sex thing; you should approach it with the same level of detail you do the 'don't trust strangers' conversation. I knew that nobody should touch my private parts and mouth kissing was for grown-ups, like coffee, and if anybody tried, I should go find other adults + tell them. I couldn't have told you anything about why somebody would do that; I just knew there were bad grown ups, and that's about the level you're working with at 3 years old. There are bad grown ups (like whatever bad person in TV shows she watches), and if you find one, go get a good grown up right away. Emphasize most people are good and it's very rare, like how everybody in the family practices for a fire, and that we all watch out for one another. That sort of thing. (I don't know your parenting philosophy.)

I'm assuming you have a neurotypical, potty-trained 3 year old; it's more complex when things like toilet-help are involved, I know.

A friend who is ED of Safe Shores (DC Children's Advocacy Center) says that Good Touch, Bad Touch has fallen out of favor. Safe Shores uses Stewards of Children, developed by Darkness to Light, and staff members are certified trainers.

Perhaps reach out to Darkness to Light or your local (regional) children's advocacy center. Many states have more than one. Not sure about the status internationally, but in the US, I know this is the case.

In my 20+ years doing public policy advocacy on a number of issues related to children, I came across too many organizations that had the potential to harm children and families, through using poor prevention models, infidelity to the model, not understanding issues, thinking that doing something was enough. While I'm generally a fan of doing something is better than doing nothing, that is absolutely not the case with this topic.

Good luck!

With my kids, my wife and I teach the idea of consent from a really young age and reinforce it strongly amongst family members. No means no as long as it's their body or their possessions

And we back that up, forcefully. If my kid doesn't want a hug, they will not be getting a hug. If they don't want to share, they won't.

My kid's autonomy is worth way more than those relationships. Period.

And it's worth it. Your kids will naturally share when they feel comfortable doing so. They'll share affection when they feel comfortable. And longer term this extends into topics they're not mature enough to tackle at 3