Running a job on a v3 agent from v5 MP

As we are upgrading our scheduler, I would like to migrate jobs one by one from v3 to v5. Here are steps I've taken so far:

1. Installed v3 on server A and restored latest copy of job database there.
2. Installed v5 on server A and went through the conversion process. All jobs are now fully converted on server A
3. v3 is still running on server B.
4. Set up a remote agent on v5 that references the v3 on server B.
5. Changed one of the jobs on v5 to run on the v3 remote agent host. Used the same service account that's running on server B as the job authentication.

With the above setup, I thought the job should run with no issues. That's not the case. On the job log on server A, I received the error: "user authentication failed". What user account does it need? The v3 instance does not have any job database user configured. Is that the issue?

It uses Windows user account in this case. Please be sure the user name is entered in domain\user format. If using a local user, you can specify .\user format or machine\user format.

OK, I took that step and I no longer receive authentication error. However, the job still aborts and now the error is "Remote Job Failed. Exit code: -1;"

I checked both scheduler logs from server A and Server B. Server A just has the generic error and Server B doesn't even register that the job attempted to run so my
guess is it never made it to the agent host?