<aura:handler name="init" value="{!this}" action="{!c.init}" />
    <aura:attribute name="gridColumns" type="List" access="PRIVATE" />
    <aura:attribute name="gridData" type="Object" access="PRIVATE" />
    <aura:attribute name="gridExpandedRows" type="List" access="PRIVATE" />   
    
    <div>
        <lightning:treeGrid
                            columns="{! v.gridColumns }"
                            data="{! v.gridData }"
                            expandedRows="{! v.gridExpandedRows }"
                            keyField="name"
                            />
    </div>

//Controller
({
    init : function(component, event, helper) {
        
        component.set('v.gridColumns', [
            {label: 'name', fieldName: 'name', type: 'text'},
            {label: 'Phone', fieldName: 'label', type: 'text'},
        ]);
            helper.getData(component);
            
         }
  });

//Helper

({
	getData : function (cmp) {
        var action = cmp.get("c.getTreeGridData");
        action.setCallback(this, function(response) {
            var state = response.getState();
            if (state === "SUCCESS") {
                var data = response.getReturnValue();
                var temojson = JSON.parse(JSON.stringify(data).split('items').join('_children'));
                console.log(temojson);
                cmp.set('v.gridData', JSON.parse(temojson));
            }
            // error handling when state is "INCOMPLETE" or "ERROR"
        });
        $A.enqueueAction(action);
    }
})

@AuraEnabled
    public static String getTreeGridData(){
        List<Account> accs = [Select Id , Name,Phone,(Select Id , Name,Phone from Contacts) from Account];
        Map<Id , Contact> opps =new Map<Id , Contact>( [Select Id , Name,Phone,(Select Id ,Name From Opportunities) from Contact]);
        
        List<AccountWrapper> aooo = new List<AccountWrapper>();
        for(Account a : accs){
            AccountWrapper aWraper = new AccountWrapper() ; 
            aWraper.name =a.Name ;
            aWraper.label =a.Phone ;
            List<Items> co = new List<Items>();
            for(Contact c : a.Contacts){
                Items conWrapp = new Items();
                conWrapp.name =c.Name ;
                conWrapp.label =c.Phone ;
                
                List<Items> wrapperOooo = new List<Items>();
                for(Opportunity o : opps.get(c.Id).Opportunities){
                    Items ooo = new Items(); 
                    ooo.name = o.Name ;
                    ooo.label = o.Name ;
                    wrapperOooo.add(ooo);
                }
                
                conWrapp.items =wrapperOooo ;
                co.add(conWrapp);
            }
            aWraper.items = co;
            aooo.add(aWraper);
            
        }
        return JSON.serializePretty(aooo) ;
    } 
    public Class AccountWrapper{
        @AuraEnabled
        public String name {get;set;}
        @AuraEnabled
        public String label {get;set;}
        @AuraEnabled
        public List<Items> items {get;set;}
    }
    public Class Items{
        @AuraEnabled
        public String name {get;set;}
        @AuraEnabled
        public String label {get;set;}
        @AuraEnabled
        public List<Items> items {get;set;}
    }

Output:


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