Gareth Southgate has experimented by using Luke Shaw as a third centre back in training as he works on alternative formations before Sunday's European Championship opener against Croatia.
England played with a back four in their warm-up matches against Austria and Romania last week but a 3-4-3 system remains an option as Southgate fine-tunes his plans for the tournament.
His deployment of Shaw was the most intriguing development in training on Wednesday because the Manchester United left back has not played at centre back for his country. It is also indicative of England's limited options.