mail -high sierra does not show 3 columns

Hello

dont know what i did but suddenly my mailprogram is only showing 2 columns, baisically I am missing the 3rd colum where i can read the eamail. at the moment i have to double click in one of the emails on the list in the 2nd column so a new window opens and than i can read the email but i would like to go back to the 3 column layout. Thank you for the help