In regards to Alien 3, there are only two scenes with CGI...the head crack and some of the debris flying around outside, I believe there may be a third but it escapes me now. What most people consider bad CGI is actually good old fashioned practical effects and green screen which people always champion as being superior to CGI. The Alien is always either a rod puppet, a miniature or good old fashioned guy in a suit.
Watch the assembly cut, the theatrical version cuts out a huge mid part and makes little sense. Disregard the opening, as it moots a lot of the previous movie and it's a fairly good movie. Not a patch on the prior two but doesn't deserve the hate heaped upon it. It totally fits the bleak, dirty future portrayed from the first movie onwards.
There's also some great performances, Charles Dance is just excellent.