Power bank adaptors that fit Mavic 2 Pro batteries have been available for quite a while, but I've only just now found an adaptor that works with Mavic 3 batteries: the Symik Mavic 3 2-way charger-discharger. It's available from Amazon and currently sells for $54.99.

The adaptor holds a single Mavic 3 battery and has one USB C port.  It can be used in either charger or power bank mode; however, I prefer to use only DJI chargers as a matter of safety. A fully charged Mavic 3 battery with this item in power bank mode is strong enough to easily power and charge a laptop, as well as smartphones, tablets, and similar devices.

dupe:  https://forum.dji.com/forum.php?mod=viewthread&tid=276366

I think I saw that one before. Looks great.

Hi there. Thank you for reaching out. Hope that you could get the best recommendation from our fellow DJI Pilots here about the said accessory. Should you have inquiries or concerns, please don't hesitate to reach us. Thank you for your valued support.

The Saint Posted at 10-9 21:31
dupe:  https://forum.dji.com/forum.php?mod=viewthread&tid=276366

Sorry for the duplicate posting. I hadn't seen the post you shared.

NDI-Orlando Posted at 10-10 07:31
Sorry for the duplicate posting. I hadn't seen the post you shared.

no big deal.  i would have bought this item regardless who posted it or when i had seen it.  such a compelling piece of equipment....plus amazon had a 10% off coupon.

Got mine and love it!