The TC Helicon VoiceLive 3 and OnSong are fully compatible with one another via MIDI.
Check out this short video to learn how to use OnSong's Bluetooth MIDI feature to connect the VL3 to OnSong, as well as how to use OnSong to change patches on the VL3: https://www.youtube.com/watch?v=NtwXp0IaW10&t=149s
Here are some additional resources that might be useful to you in connecting the VL3 with OnSong in conjunction with other products, like the BeatBuddy: