we didn't use baby sign language. I tried it, but I kept forgetting to use the signs consistently enough.
It's important to remember that your child being trilingual will NOT slow his development, per se. It will just divide his vocabulary into three languages, which might make it harder for him to gather enough together in any one language to ask for things. But children are extremely clever and resourceful and they will figure out ways to make you understand them and even if you don't use sign language, you will learn their signs. For example, we;re raising our son with 3 languages and when he was little we EC'd him, so from a very young age he would do this dance when he needed to poop and it was impossible to mistake. Lately he doesnn't do the dance as frequently and he's started to say "I need to poop" more instead, but he might dance still. So not official sign language, but it's just what he does.
So don't worry if you don't do sign language. It might be helpful to give them another non-verbal way to communicate, but it's not necessary by any means.