The Ripper Street S2 finale aired on April 12 on BBC America.
Keep in mind that while BBC One cancelled Ripper Street, BBC America did not, and is co-producing S3 with Amazon. From the articles I read, S3 will definitely premiere on Amazon in the UK before it possibly airs on UK television. However, in the United States, that might not be the case. My guess is that S3 premieres on BBC America first, then Amazon soon after (possibly even within a week of the TV broadcast).