Rabbits do best in pairs. Introducing a 3rd rabbit doesn't always work. Trios can be difficult to achieve. That said, it's surprising the bonded pair have been so hostile toward the baby. Often babies are accepted by adult rabbits... at least until the babe's hormones kick in. Then all heck can break loose. This is why it isn't advised to try to introduce a baby to an adult rabbit (even a single adult).
Once the little one's hormones kick in, things may become even more difficult. As this one matures, the bonded pair will see it differently. Sometimes the presence of a third rabbit can break the bond of an existing pair. So you may have a difficult road ahead.
There is a good resource on how to go about trying to establish a trio. But attempts really should not begin until after your baby is fixed. Then you'll need to wait about 2- 4 weeks after surgery to begin any bonding.
Here is that resource. There is a contents page on the right of that link. Go to #7 for
Rabbit Groups & Trios