I was successfull in making Designer start in Pharo 3.0 30659, with some help from 

Tobias Pape. 

The following changes were needed (compared to what was needed for Pharo 2.0)

- floodfill2:at: is gone from Form. I replaced it with a simple fillColor: 

  That it had gone missing was not clear from issue 11570

- outsetBy: is deprecated and replaced by expandBy: in a lot of places

- (some symbol or string or something else, #:) asSymbol is replaced by ... asMutator

  in a lot of places.

- TranslucentColor is replaced by Color

TextMorph handleInteraction:forEvent: no longer exists, but gives a different problem in Pharo 2 than in 3

just replacing it by handleInteraction: does not work. That makes a mess of the selection in

theUiTextLineMorphs in the object inspector.

Stephan