Can you send her some AA leaflets? There's a checklist on there which anyone answering honestly will have to confront that they have a problem. Really don't know what to advise otherwise. The help available isn't much, but the three year old is obviously in danger.
The other thing, is that it's not your problem. Sounds harsh, but you need to look after yourself and alcoholics are very good at leaning on other people to take on their burdens. If you really feel you have to do something, I'd second what others have said and call Al-Anon, explaining your worries to them and asking them for advice - take your lead from them.
Awful situation. A friend of mine was an alcoholic for a while when her children were growing up, but they were slightly older than 3, and the oldest one was a typical child of an alcoholic - she ran the family from the age of about 8. I never felt any real worry about danger, because by the time I realised she was an alcoholic, the older one was about 12 and very capable, but otoh her sister wasn't three anymore. And my friend knew she was an alcoholic, so did go to meetings and try to control her drinking.