1) Yes. You can use Automic File Transfer (Agent to Agent) or the RA FTP Agent. 2) No specifically relevant information or configuration is required, outside of obtaining the appropriate OS Agents or FTP Agent.
3a) If the API is web services, we should be able to call this via our REST/SOAP Web Service Agents. 3b) If not, but the API can be called from command line, this should also be able to be accomplished. We wouldn't be able to provide guarantees from our side regarding the API functionality, however - just whether we executed the correct commands successfully/correctly in this case.
Noted behavior with Amazon S3:
It has been reported and experienced that files on Amazon S3 may not update immediately as on a regular file system, after a file transfer. The delay was not very long (within a minute or so), but if there are File Checking Events that have strict times for execution and checking this is important to be aware of.