Find someone with a non-updated S5 and the older version of SVoice. Install App Backup & Restore on the S5 (just for now, you can uninstall it when you're finished). Back up SVoice. You'll get an apk file of it in the App_Backup_Restore folder. Uninstall SVoice in the S7 (or use AB&R to save an apk of it. Copy the S5 apk to the S7 and run it (find it with a file manager and tap it). It'll install in the S7. (You can't install an older version over a newer version, which is why you have to uninstall it first.)
Which is the best of the 3? That's a personal decision - if you prefer the S5 version of SVoice, that's the best one for you. (I don't use any of them, so I have no opinion.)