Here is my code, but it won't work. Following are the cases and assume phone numbers are same.
Case 1:
Lead1(Existing Lead) -
Field1 - Null
Field2 - Value 2
Field3 - Null

Lead2(In TriggerList) - 
Field1 - Value1
Field2 - Null
Field3 - Value 3

Lead3(In Same TriggerList as Lead2) - 
Field1 - Value 5
Field2 - Value 6
Field3 - Null

When Lead is merge then Final Lead wil become:
Lead1 -
Field1 - Value 5
Field2 - Value 6
Field3 - Value 3

And Lead2 and Lead3 will delete.

public class MergeLead
{
    Integer i=0;
    Map<String, Schema.SObjectField> schemaFieldMap2 = Schema.SObjectType.Lead.fields.getMap();
    List<Lead> lstLeadToDelete = new List<Lead>();
    public static Boolean runOnce = true;
    public void onInsert(List<Lead> lstInsertLeadTrigger)
    {
        i++;
        System.debug('++++iiiiii'+i);
        if(runOnce)
        {
            InsertLead(lstInsertLeadTrigger);
        }
    }
    private void InsertLead(List<Lead> lstLeadInsert)
    {
        set<String> setPhone = new set<String>();
        Map<String,Lead> mapLead = new Map<String,Lead>();
        for(Lead objLd : lstLeadInsert)
        {
            if(!setPhone.contains(objLd.Phone))
            {
                setPhone.add(objLd.Phone);
            }            
        }
        Set<String> fieldNames = schemaFieldMap2.keySet();
        
        // Build a Dynamic Query String.
        String soqlQuery = ' SELECT ' + string.join (new List<String>(fieldNames), ',') + ' FROM Lead Where Phone =: setPhone';
        System.debug('+++++fieldNames+++++'+string.join (new List<String>(fieldNames),','));
        System.debug('++++setPhone++++'+setPhone);
        List<Lead> lstLeadToCheck = new List<Lead>();
        if(setPhone != null)
        {
            lstLeadToCheck = Database.query(soqlQuery);
        }
        System.debug('++++lstLeadToCheck++++'+ lstLeadToCheck);
        if(lstLeadToCheck.size()>0)
        {
            for(Lead objOldLead : lstLeadToCheck)
            {
                mapLead.put(objOldLead.Phone, objOldLead);
            }
        }
        System.debug('++++mapLead++++'+mapLead);
        for(Lead objLead : lstLeadInsert)
        {
            System.debug('++++mapLead.containsKey(objLead.Phone)++++'+mapLead.containsKey(objLead.Phone));
            if(mapLead.containsKey(objLead.Phone))
            {
                System.debug('+++mapLead.get(objLead.phone).id++++'+mapLead.get(objLead.phone).id);
                System.debug('++++objLead.id+++'+objLead.id);
                if(mapLead.get(objLead.phone).id != objLead.id)
                    lstLeadToDelete.add(objLead); 
                for(String strFields : schemaFieldMap2.keySet())
                {
                    if(mapLead.get(objLead.phone).get(strFields) == null && objLead.get(strFields) != null)
                    {
                        mapLead.get(objLead.phone).put(strFields,objLead.get(strFields));
                    }
                }
                System.debug('++++mapLead++++'+mapLead);
            }
        }
        if(mapLead.size()>0)
        {
            System.debug('+++++mapLead++++'+mapLead);
            update mapLead.values();
        }
        System.debug('++++lstLeadToDelete+++'+lstLeadToDelete);
        if(lstLeadToDelete.size()>0)
        {
            delete lstLeadToDelete;    
        }
        System.debug('++++lstLeadToDelete+++'+lstLeadToDelete);
    }  
}