The fluid phase reaction A + B --> R to a desired product, when accompanied by a parallel decomposition to a by-product A --> S, form a pair of reactions ideal for evaluating mixing-with-reaction tests. The reactions are typical of diazotisation in dyestuff manufacture, so that fundamental mixing studies can be linked to practical manufacturing. Earlier studies on these problems used a 2-D analysis which is approximate for some practical cases. This approach has now been extended to a full 3-D network-of-zones for a stirred vessel for single-point dip-pipe addition at any location. This is a simplified computationally tractable approach to fluid mixing, which is superior to more complex CFD solutions, because it can easily handle multiple complex reactions. The flow and mixing in the 3-D network incorporates axial and radial convection with swirling tangential flow, turbulence and flow around baffles. Computations are then tractable for long duration semi-batch addition modes of operation. Macro-mixing tin 3-D), the associated partially segregated fields of reagents A and B and the spatial-temporal evolution of the local instantaneous yields and selectivities are illustrated using solid-body AVS graphics for the Drain/ICI reactions.