What's the difference between Entity Framework (6) transactions with single and multiple SaveChanges() calls

I want to know what are the practical differences of executing a transaction in the same database context between these 3 ways:

1) Multiple operations with one single SaveChanges(), without explicitly using a sql transaction

using (TestDbContext db = new TestDbContext())
{
    // first operation
    // second operation
    db.SaveChanges();
}

2) Multiple operations with one single SaveChanges(), using a sql transaction

using (TestDbContext db = new TestDbContext())
using (DbContextTransaction trans = db.Database.BeginTransaction())
{
     // operation 1
     // operation 2
     db.SaveChanges();    
     trans.commit();
}

3) Multiple operations with multiple SaveChanges(), using a sql transaction

using (TestDbContext db = new TestDbContext())
using (DbContextTransaction trans = db.BeginTransaction())
{
     // operation 1
     db.SaveChanges();    
     // operation 2
     db.SaveChanges();

     trans.commit();
}

In (2) and (3), if commit() is supposed to actually execute requested sql queries to database, is it really different, say, save changes for each operation or save changes for all operation at once?

And if (1) can also allow multiple operations to be safely executed in the same database context so what's the main use of manually starting a transaction? I'd say we can manually provide try/catch block to roll back the transaction if something bad happens, but AFAIK, SaveChanges() also covers it, automatically, at least with SQLServer.

** UPDATED: Another thing is: Should I make db context and transaction variables class-level or these should be local to containing methods only?