My stepson was turning 6 when my daughter was born. We just told him that i was feeding his sister, and left it at that. He wasn't allowed in he room when I nursed her and that was more of we didn't want to hear things from his mom at all, about me nursing. Didn't want her to say that it was sexual in any nature so we took that precaution. (and yes that would be something she would have tried)
My daughter is now 2 and will be 2 1/2 when the next baby is born and I plan on telling her that this is how mommy's feed babies. i do pump so my husband can give the baby a bottle as well, but she will see that mommies and daddies do things differently.
Just be simple and only give the info they want to know. The 5 and 6 year old may ask more questions than the 3 year old, but i dont' think they really will ask too much.