Hi H.,
I'm French, my husband is Spanish, babysitter (at the time) and all friends speak English so our son (and now our daughter) was raised in a tri-lingual environment.
Everybody, including our pediatrician, warned us that this - even though a great opportunity for him - would probably create some speech delay at the beginning.
For us, speech delay meant that our son would not be able to communicate his needs, resulting in more frustrations, more tantrums...
We taught him sign language and it's the best thing we've ever done. We all used our own language at all time. When nursing him, I would say "milk" in French and sign i"milk" at the same time.
I think it helped him understand the languages because he had one common element he could communicate with everybody. The sign was the same for all of us, even if the word was different.
At 18 months, he would sign over 120 signs! At the beginning we were teaching him ourself by interaction (whe he was interested in a squirrel outside, we would sign and say "squirrel"). After he acquired the basic signs and the understanding of the sign language, we showed him the "Baby Signing Time" videos. With them, he was learning faster than we and it was hard to keep up!
As a result, he could always express his needs. We had very few tantrums and good communication. Some would say that the sign language created a speech delay but I think the 3 languages created the delay and sign language helped him communicate.
He actually was delayed in speech (evaluated at 14 month when he was 24 months), but when speech came in, it blossomed very fast. He is not yet 3 years old and say over 700 words in French, 500 in English and a little bit less in Spanish. And he understands even more. He can tell his ABCs, colors and shapes in the 3 languages and counts to 29 in the 3 languages.
We are now teaching our 11 months daughter.
Good luck.
Note: sign language IS NOT universal. We taught our children the ASL (American Sign Language) and I know from friends that it is different from the British sign language and from the French sign language. In our case, we discovered sign language and learned with our son, so we chose ASL (because we live in the US, where some schools/daycare use it)