Ditto Momma L's excellent answer. Your 3.5 yo has demonstrated he cannot be trusted when unsupervised, he should no longer be allowed to be.
I had a problem with my nephew and my little guy, who is now 2.5. He would cry or scream or have mysterious injuries, couldn't tell me anything, but his cousin knew nothing and he was the only one with him. We even had to take a trip to the ER last year for stitches because he fell and split his forehead open, but my nephew, the only witness, couldn't tell us what happened, and he was 9 at the time. We stopped allowing the boys to be alone anymore, and magically the crying and injuries to my guy stopped.
When he turned 2 it all changed as he can now tell on his nephew, who's getting away with less and less.