can an iron swing weight of D3 be achieved with a light weight 90 gram shaft?
Sure, if the Swingweight is too low use lighter grips. If the Swingweight is too high add a backweight. Please note that Swingweight is not a useful way of matching golf clubs. You may consider replacing Swingweight with BioMatch, one-swing golf club matching, at www.rational-golf.com.
A few words on Swingweight. In the 1920s, the concept of Swingweight was introduced as a simple-to-understand approximation of MOI (moment of inertia) matching. Along with it, a device was created for making Swingweight measurements called a Swingweight Scale to enable club makers to build golf clubs to a specific Swingweight measurement. If a set of clubs having the exact same grips and completely identical shafts trimmed incrementally, are matched by Swingweight using only the club head for making adjustments, then the MOI of each club will be reasonably matched. This was the original intent of the Swingweight process. When the Swingweight method was introduced it had some credibility as all the shafts in the set of clubs at the time were made out of wood. However, today, with the more modern shafts, that have purposeful variation along their length, and the tendency to mix and match a variety of shafts within a single set, the less likely it is that a Swingweight matched set is relevant as a method of matching golf clubs.