What does that mean? :3

Hello!

As a faithful user of :3, I'm adding my two cents, especially since the meaning has evolved quite a bit since the subject was created (I've felt it myself!):

Initially, yes, ":3" is inherited from kawaii culture and had a "cute tone" or "a bit playful" meaning like: "I ate the last cookie :3," if we consider a period up to around 2019 in my opinion.

It has evolved more among younger users, and by capillary action, it has also evolved among older users towards a more intimate/cuddly meaning, showing more of a sign of cute attachment or a hint of flirtation like "Oh, I wanted to see that movie too :3," "I'd love a pizza :3."

You get the idea!
However, be careful, as with all emojis, the meaning is highly dependent on the person and could have a very particular meaning, perhaps even remaining in the kawaii context if your interlocutor is immersed in it!