public void Merge( 
   EditingRowTemplate template,
   Layer sourceLayer,
   IEnumerable<long> sourceOIDs
)

// Merges multiple features from a source feature layer into a new feature, optionally using a template or inspector, and supports merging into a destination layer.
await QueuedTask.Run(() =>
{
  var mergeFeatures = new EditOperation() { Name = "Merge Features" };

  //Merge three features into a new feature using defaults
  //defined in the current template
  mergeFeatures.Merge(currentTemplate as EditingRowTemplate, featureLayer, [10, 96, 12]);

  //Merge three features into a new feature in the destination layer
  mergeFeatures.Merge(destinationLayer, featureLayer, [10, 96, 12]);

  //Use an inspector to set the new attributes of the merged feature
  var inspector = new Inspector();
  inspector.Load(featureLayer, objectId);//base attributes on an existing feature
  inspector["NAME"] = "New name";
  inspector["DESCRIPTION"] = "New description";

  //Merge features into a new feature in the same layer using the
  //defaults set in the inspector
  mergeFeatures.Merge(featureLayer, [10, 96, 12], inspector);

  //Execute to execute the operation
  //Must be called within QueuedTask.Run
  if (!mergeFeatures.IsEmpty)
  {
    //Execute and ExecuteAsync will return true if the operation was successful and false if not
    var result = mergeFeatures.Execute();
    //or use async flavor
    //await mergeFeatures.ExecuteAsync();
  }
});