A.H.
There are several books out there on bilingualism in children and language acquisition. We did a lot of reading before our first child, as we had decided to raise our children English/German bilingual. Our favorite book was Bilingual Children from Birth to Teens by George Saunders. (You can get it out of the library on interlibrary loan if you can't find the book easily.)
My husband is the German speaker, so he only speaks German to the kids. I know a little German, but not enough to teach them well, so I only speak English to the kids. (I do know enough German that I can understand what my kids are saying to my husband and what he is saying to them.)
Our kids are 6 and 4 now and doing great with both languages. It has not hurt my daughter (age 6) to know German and I think her reading in both languages is about the same level in terms of ability. She has absolutely no problems in school with English at all.
Unfortunately, I have no experience with adding a third language although we do have friends who were doing Spanish/English bilingual and have now moved to a French-speaking country and their kids, ages 4 and 6, are picking up French in addition to the Spanish and English they get at home.
We have several friends who speak only German to their kids at home and they have picked up English easily when it was time to go to preschool or school.
Allowing your child to grow up learning more than one language gives them many opportunities.
Good luck!