Ask Mr. Smarty Plants

Diagnosing "sad" even if we could see it is not easy, so we will give you some clues, and perhaps you can go from there. Morella cerifera (Wax myrtle), which we frequently recommend for Central Texas, is more comfortable a little further east in Texas, as you can see from this USDA Plant Profile Map, but still does very well in the Austin area. What you need to ask yourself is what is different in the sun exposure, watering or soil between the two healthy shrubs and the sad one. Here are the Growing Conditions from our webpage on wax myrtle:

"Water Use: High
Light Requirement: Sun , Part Shade
Soil Moisture: Wet , Moist
Soil pH: Acidic (pH<6.8) , Circumneutral (pH 6.8-7.2)
CaCO3 Tolerance: Medium
Drought Tolerance: Medium
Heat Tolerant: yes
Soil Description: Slightly acidic, moist, deep sands, loams, clays.
Conditions Comments: Requires constant moisture to get established, but both drought- and flood-tolerant once established. If temperature goes below zero degrees F, will defoliate, not releafing until spring. Tolerant of saline conditions and urban confinement within pavement."

We consider "sun" to be 6 hours or more of sun a day, and "part shade" 2 to 6 hours. Is the unhealthy plant consistently getting less than 2 hours a day? Wax myrtles need a very slightly higher amount of acidity than is normally in our alkaline soils, but if the healthier ones are prospering in your soils, we can think of no reason why the third one should not. Watering trees by sprinkler system is not our favorite choice but, again, the other trees seem to be okay. We would check, since this plant calls for high water use, just how much water is actually getting to the tree in question. If you feel, for any reason, it is drier, we would suggest pushing a hose down into the soil around the roots and letting it dribble slowly until water comes to the surface and doing so twice a week for a while. Just simply because of the position of the sprinklers, that particular tree may not be getting sufficient moisture in this extreme drought situation in which we find ourselves. Do not fertilize. You never fertilize a plant under stress-the fertilizer will be trying to force more growth out of a plant struggling to survive. 

Finally, there may be a situation over which you have no control: perhaps the roots have hit a high concentration of rock, there could have been severe bark or root damage caused by some sort of digging or mowing, or below-ground pipe installations or roots trying to get through a paving or foundation. There is one more possibility in our list of problems - fusarium wilt. Please read this previous Mr. Smarty Plants answer to explore that possibility.