Bf 109E-1 and E-3
 

I have read that the Bf 109E-3 was only just entering squadron service as the war began. However my new book 'German Aircraft Industry and Production 1933-1945' by Vajda and Dancey states that by 31/12/38 15 Bf 109E-1s and 153 Bf 109E-3s had been delivered. By 31/8/39 522 E-1s and 603E-3s had been delivered. Since 631 Bf 109Es were on first line strength 31/8/39 I presume that both types entered squadron service at the same time. Can anybody comment on this?
Were units equipped solely with 1 type or were E-1s and E-3s mixed together?
I understand that E-1s were subsequently brought up to E-3 standard. When did this upgrade start and how widespread was it?

Thanks,

Neil