Loading the song doesn’t start playback. To start playback you need to decide what MIDI command you want to send from Ableton. I use a C1 note on. Then you need to teach ST3 to recognise your chosen MIDI command (IE note on) as the Start Playback command. You will find the settings in Settings/Assign Action.
So to summarise.
Create a MIDI track in Ableton
Set the iPad Network Connection in the MIDI out setting
Set the Channel
Create a clip on the MIDI track
Add a note (if like me you use a Note On command to start playback)
Set the note to a value. I use C1
In ST now, go to Settings/Assign Actions. Press learn next to Start Playback
Press play in Ableton to play your midi track across the Note on clip
ST should see the command and learn it as the Start Playback command.
Now whenever you send a C1 Note on from Ableton, playback will Start in ST
Hope that helps.