I was recently asked by the mom of a 3 yr old girl how to approach the topic of death with her. They had no religious affiliation or particular personal beliefs regarding death. Do you have any advice on how to tell a 3 yr old about death and any good reading suggestions for both mom and me?? Thanks!!

It's a hard question, especially without a little background. Surely she has a specific reason for asking. I'd guess first you should ask in what context it came up. It's much easier to answer for a specific situation. And always, when a 3 year old asks a hard question, I would say "What does that word mean?" or "What do you think?" It is extremely common for their question to be about something much less than the adult hears it to be. It might just be about what happened to the bad witch at the end of the fairy tale.
At any age, the first issues about death are about separation, security, being abandoned. Three year olds are egocentric ("If you die, who will feed me?") and concrete in their interpretation of most explanations. They don't understand concepts of relative time ("Very old" could apply to their 12 year old brother.) The whole thing may come undone at a later time. Definitely avoid "Going to sleep forever" or they'll be back sleeping with you forever. They can think in stories or parables, which give them something to tell themselves, although that isn't a particularly deep understanding.
Gerald Koocher has written about it, including a little in Levine's Developmental-Behavioral textbook. His best point is that adults don't understand death either. You might take a look at Stein and Dixon's Encounters with Children. There are a lot of works of fiction on this topic. Also a lot of didactic children's books, which can be useful or not, depending on the story the parents want to tell.
It's almost impossible to do this without a story, and religion provides the traditional stories that work at this age. Any other story will do, but the parents should compare notes before it goes too far. In this culture, they will all hear their agemates' stories pretty quickly.
They rely on magical thinking and mirror the words and feelings of their parents. Kids of 3 don't really understand cause and effect, so they may think being bad or wishing or being near old people can cause death. By 3 1/2 or 4 they start to count and sequence, and to guess that in years, you can't rewind. They know death is physical, which fits right in with the body integrity worries they have anyway at 3,4 and 5.
They absolutely cannot be fooled or protected by having secrets kept from them, but they do not need to hear adults explore all their own anxieties. Their concerns will be about their security and about physical, concrete things, and they may tolerate the inevitable contradictions better than the adults expect.
There's a book called "Life and Loss: A guide to help Grieving Children," by Linda Goldman, which is for parents, but I got a lot out of it. It talks about saving pictures that commemorate the child's relationship with the dead person, keeping a "memory book' or box with momentos to hold on to the thoughts so the child doesn't need to struggle to keep the memory in mind, writing (or dictating) letters to express and remember the feelings, the need to process the loss again at subsequent developmental stages...Very practical.
Start simple, follow the kid's lead and don't feel you have to wrap up a complete solution in one sitting.
I hope this helps. Yours, Harriet McGurk