Migration of 3.7 SP3 project is not possible. What you need to do is to upgrade your project to 3.7 SP4 first and then to migrate it to the latest and greatest version of Sitefinity. In order to upgrade your project to SP4 you have to follow the steps from the following documentation article:
Upgrade projects to Sitefinity 3.7
When the project is upgraded to version 3.7 SP4 you can start migrations process as outlined in the following documentation article:
Migrate Sitefinity 3.7 projects
Version 8.0 of Sitefinity will be released this week so I would advise to consider migration directly to this version.
I hope that you will find this information helpful.