how to convert SQL join query to Entity Framework in C#?

You can try this command using Entity framework (.edmx model), i suppose you have created .edmx and mapped your database with the application(this is half solution)

using (var ctx = new EdmxContext())
        {
            var query = ctx.Office
                .Include(a=>a.Rate)// Your Rate table id is linked with Office table using reltionship
                .Include(a=>a.Splash) // Your Rate table id is linked with Office table using reltionship
                .GroupBy(a=>a.Office.id).OrderByDescending(a=>a.Value).ToList(); 

            

        }

Above is half solution, to properly check your page table details, please provide database relationship image, with example data, i test it here and try to give bettter reply

Or check these links for taking out Average

Another link of average example

For reference of EF

You can run your query using Entity framework also, with the help of .SQLQuery() method, something like this

  var query = "SELECT * FROM table_name";
  var queryResult=Context.Table_Name.SqlQuery(query).ToList();

it may help you.