The unsafe thing about it is all three of them are "downers" , CNS depressants. Mixing these increases the risk of respertory depression among other things.
To answer your question I think it was the diazapam more than anything. Benzodiazepines tend to overwhelm opiates if you take to much. You usually have to take a dose lower than your regular dose
Once when I still had a low tolerance I took 30 mg of oxycodone. At the time This dose would have normally gotten me a VERY nice opiate buzz, not quite nodding however . Well ontop of the percocets I also took 2 mg of alprazolam. This dose of Xanax would get me content, but I usually needed 3-4 mg to be wasted.
To make a long story short I could not even feel anything resembling an opiate buzz, but I could tell the percocet was contributing to the sedative properties of the Xanax. I felt as if I had taken 4 mg instead of 2 and boy was I messed up. But I prefer opiates to benzos so I was upset as well that I missed out. So as you can see benzos such ad Valium, Xanax and klonopin can majorly interfere with a good opiate high.
I hope this is helpful. Be safe brother!