It seems to me that if you have a cable from the OM3 to your phone, in order to charge the phone while filming, the cable is going to kind of drag on the gimbal motors and generally unbalance the phone.  But I've not seen any advice from DJI on that.  What's the official recommendation - is that ok, or wait till the gimbal is off?

Hello and good day. Thank you for the inquiry. The feature of the DJI Osmo Mobile 3 ( charging the phone from the DJI Osmo Mobile 3 when in use ) was tested by DJI to work properly when using the said gimbal. In addition please connect your mobile phone to the USB-A port. If The DJI Osmo Mobile 3 is powered on, the mobile phone will begin to charge. If the DJI Osmo Mobile 3 is powered off, please press the M button once to start charging your mobile device. Thank you.

Obvs. not official coming from me, but my two pennies worth is... it kinda works.  But in my experience the standard USB cable that comes with most phones gets in the way and hits the body or arms of the OM3 if you're shooting in any mode other than landscape with very little movement.  I can't change from landscape to portrait with a standard iPhone lightning cable attached for example as the white lead hits the top of the OM3 handle.  It just makes using the gimbal uncomfortable and more difficult.  I've taken to only using the gimbal as a power bank to charge my phone when I'm not shooting.

But as long as the lead isn't making contact with the gimbal I haven't come across any unbalancing issues as long as long as the phone is perfectly balanced on initial setup.

Thanks.  Personally I don't expect to have to charge the phone in this way other than very rarely but it's good to know it's deemed to be ok.