I was diagnosed with Stage II Hodgkins Lymphoma 10 weeks after the birth of my second child. It is devastating. But, the MOST important thing you can do is be there. The only thing I needed was the reassurance that people wanted me in their lives, and that gave me the courage to fight.
Her prognosis may not be catastrophic. A friend, whose mother just had a recurrence of breast cancer after 12 years clean, has the same diagnosis (though I don't know the exact kind of breast cancer). She has been told she has MANY good years ahead of her.
There are a lot of support groups for family members who are caregivers and face being the ones left behind if the cancer is terminal. The one I'm involved in is Imerman Angels (imermanangels.com). Caring Bridge is also another fantastic place that I'd recommend getting involved in.
I'd also recommend getting second and third medical opinions. Part of the way through my treatment, I asked to be referred to additional Lymphoma specialists (I had to go out of state), but I actually chose not to follow my own Oncologist's recommended treatment based upon those consults. It never hurts to have those additional opinions to help either confirm the prognosis/treatment or to open the opportunity for other options.
As for your 3 year old - please be honest about what's going on. My diagnosis was the day before my son's second birthday. We took him to chemo sessions (because we don't have family close by), explained to him that mommy was sick and tried to explain that it was different from when he gets sick. They don't need to know the nitty gritty details, but it will help prepare them for possible mood/physical changes that may come with treatment or how other people deal with it.
If nothing else, be there for her, for your father and your family. Everyone wants to help initially, but it's the months later when people hear you're well that they stop calling - and you probably need them the most.
Best wishes to your family.