EF 6 Database First Approach multiple connection string issue

Hi !! 

We use database first approach. We have about 3 different database and each with multiple DbContext. Managing connection string per environment for each DbContext is a headache. Is there a simpler way we can have Single connection string for each Database which can be used in respective DbContext. 

e.g.

Database1

DbContext11, DbContext12, DbContext13

Database2

DbContext21, DbContext22

Database3

DbContext31, DbContext32