The error you have there seems to indicate trying to install a version from before 3.0.0.M2 and some of the components that it depends on such as 'com.springsource.sts.frameworks.ui' are probably no longer available from snapshot update sites.
3.0.0.M2 was actaully released fairly recently and it is based on Eclipse 4.2. So you could try to download the new 'Groovy Grails Tools Suite' which is a prepackaged version of STS with the groovy grails tools already included.
You can get it from here:
Go to the other downloads page and scroll down to find GGTS 3.0.0.M2 there.
I think that's probably the easiest route at the moment to get an Eclipse 4.2 based STS with Grails tools installed.
Also its advisable when switcing to a post 3.0.0.M2 version to do via a clean install rather than an update of a pre-existing install because the bundles and package names in 3.0.0.M2 have all changed which may cause chaos in update scenarios (e.g. having both an old an new version installed at the same time).