Q.wax myrtle
Planted 10 wax myrtles 5 years ago and they did well. Suddenly 3 have died completely and others are struggling with some new growth. What can be done to help continued growth?

Is this the common wax myrtle or the dwarf variety? It would help my diagnosis of your problem if you could provide additional information on what is happening to the trees. Anything unusual about the leaves, bark, etc.? Any signs of insect infestations? Has the demise followed some cold harsh weather? Tell me about your care of the tree in terms of watering, fertilizing and pruning.
To help continued growth, I would recommend you follow the guidelines in the following articles:
https://www.gardeningknowhow.com/ornamental/trees/wax-myrtle/