QGIS Variable Symbol Levels / Layer Rendering

From a cartographic POV your idea is at least troublesome. This could lead to too much information present in the map, and also to collissions if you have identical values for two attributes.

Option 1: Use diagrams in addition to points.

Option 2: Add the values as textlabels.

Option 3: Work with patterns/transparency instead of shuffling the symbol-order all the time.

Option 4: If it has to be your way, you can solve this by using rule-based styling. For each possible combination of the three values create a rule like "A">="B">="C", then have each marker consist of three separate markers, which you must order so the smallest values comes out on top (in this example-rule C). There still is the problem of identical values.