Searching for ‘V3 documentation’ leads you to: Where to find the V3 documentation?
In the V3 documentation search for ‘payload’.
Payload Formatters is listed on the Integrations page.
Some things have changed e.g. instead of of bytes and port now use input.bytes and input.fPort. See documentation for more changes.
IIRC existing V2 payload functions should also work in V3 for backwards compatibility but I have not tested that.
Next time please do some searching yourself first.
FYI, also see About the V2 to V3 Upgrade category