I literally haven't watched Robocop 3 in 30 years, but in my 10 year old mind it wasn't horrible.
If I watched it again, I'd probably agree its horrible.
But Robocop 2 wasn't a great sequel either, so I imagine expectations weren't very high for it to be great.
Terminator 3, on the other hand, had two of the greatest SciFi/Action movies of all time proceeding it, so expectations were sky high.... and it crashed so hard its considered one of the worst sequels ever.
But that's considering the expectations & hype before the movies were released. I'd have to rewatch Robocop 3 & Terminator 3 to have a more objective point of view on both movies.... and I really don't want to watch either.