To the OP, are you just presuming that both your OPN 3 and your Airpods Pro should play simultaneously on the same content if you wear both at the same time and play music or videos on your iPhone? Or did somebody from Apple say that it can be done?
I have the OPN 1 and the first gen Airpods. I can tell you that even though the OPN interfaces with the iPhone MFI, that’s just an interface, but it still goes through Bluetooth (albeit Bluetooth Low Energy proprietary to Apple iOS only), and you need to have Bluetooth enabled on your iPhone in order for your OPN to work.
I also own a ConnectClip with my OPN. I can use it on my Samsung Note 8 and connect both the ConnectClip and the Airpods to the Note 8 and play the audio content SIMULTANEOUSLY. Unfortunately they have different lags, otherwise it would have been great for me to get the bass from my Airpods and the highs from the OPN via the ConnectClip.
But if I try the same setup with the iPhone, not through MFI but through the ConnectClip, I can connect to both the ConnectClip and the Airpods at the same time. HOWEVER, they don’t play simultaneously like on the Note 8. If I put on the Airpods, then only the Airpods play. If I take off the Airpods, then the music is paused, and if I click Play, then I hear from my OPN through the ConnectClip. But I don’t have to connect/reconnect, they both remain connected at the same time, they just don’t make the sounds simultaneously like with the Note 8.
Now if I pair the OPN via MFI (and not with the ConnectClip), the same thing also happens on the iPhone. They’re both connected, but only either the Airpods play when I put them into my ears, or when I take them out, the music is paused and if I click Play, the OPN starts making the sound.