Thanks for all the inspirations guys.

What I've settled on is a pseudo-3D field, with tridents (3-point stars or crosses) with the length of each point representing one of the outputs: thus.

The inputs (axes) were obvious candidates for log₂ scaling.

I tried various methods of reducing the range of the values; and settled upon the simple 'difference from minimum' as giving the best visual representation of the relative values, whilst minimising clutter.

By aligning the three star points with the x, y & z axes, it made it easy to visually distinguish aligned points in 3D, without the need to add extended ticks or grid lines, which tend to clutter the graph.

I did consider adding a line or curve connecting the end points of the tridents in each plane; but I couldn't wrap my head around how to code that.

• I tried tye's R=mem, G=Insert, B=lookup color blob: but it is impossible to order a muddy brown blob, a muddy purple blob and a muddy orange blob visually.

  But using 3 primary colours for the 3 output values was a nice idea.

• I tried LanX's cuboid: but the area of each face becomes the product of two different measures; so the visual cues as to relative size get muddled.

  But plotting the three outputs in different planes was the basis of the tridents.

• I tried Davies stacked bar charts; but on a 2-D plot, whichever way I oriented them, smaller ones ended up hidden behind bigger ones no matter how I tilted or rotated the view.

  And grouping the 3rd dimension made for skinny tall bars that were difficult to read.

  I also tried floating the stacked bars in a 3D field: but still, however I rotated the view, some bars obscured other unless I made them so small they were unreadable.

  Also, comparing the relative values of bars (other than the datum aligned (bottom) one) becomes very difficult.

  But keeping the three outputs distinct by colour; but putting them together to give an combined size cue, works.

Once again, thanks to all who responded.