Well, an additive matrix model is XYZ = RGB * M, and solving for M
requires only 3 linearly independent (RGB,XZY) pairs. You need the
"complete" XYZ of the primaries though (including their luminance, and
not just the chromaticity). But measurements include the luminance
anyway. And white is just the sum of the XYZ of the primaries, so it can
be computed from the primaries [although of course in practice, displays
may not be aware of this rule and the measured primaries may not sum up
exactly to the measured white. Still, if the measured white isn't
reasonably close to the sum, one's alarm bells should ring.].
