The only way I can imagine a scenario where it was necessary to shoot all three is if they were all attacking at once. While that CAN happen, it just seems unlikely, but as said there isn't enough to go on yet.
Maybe he used his command voice and they didn't do what he said.
1. One or more parents could have been behind the 32 year old.
2. One or more may have jumped in front of him protectively. Or since he was mentally disabled, he may have tried to hide.
3. If they were next to him, the officer may just be a bad shot rattling off 7 or 8 rounds as quick as he could.
4. Could be one of the parents tried to stop him from shooting.
If there is video and it gets released, that will answer most of my questions. If this were a non-cop, video or witness statements would be needed to back up their statements to prove it was self defense. In this case, I don't know. The article hinted that witnesses may not back up the officer's story, but they didn't quote anyone.