The Shimano steps chargers that I see for sale don't have the 3 pin plug that is on my YT's battery charging port, is there an adapter available that will fit the YTD 3-pin port?
You caught something I got wrong - I said that "3-pin" plug was a standard Shimano STEPS EC-E6002 port. It isn't. I've dug back into this and BikeRadar's Decoy review confirms it plainly: the YT battery uses its own charger with a proprietary port, meaning standard Shimano STEPS chargers can't be used to charge the Decoy.
So the pin count and voltage looking Shimano-ish was a red herring - YT built their own charging port for the custom battery (that 540Wh pack made for them by SIMPLO), even though the drive unit inside is Shimano's EP8. Sorry for sending you shopping for the wrong part.
That means what you actually need is a genuine YT charger, not an adapter into a Shimano one - there's no adapter that'll make a Shimano charger talk to that proprietary port, because the plug shape and pin layout are YT's own design, not a Shimano standard with a different shell.
Where to actually get it:
• Go straight to YT Industries' own spares/customer service channel - as a direct-to-consumer brand, YT don't rely on shops like Jenson to stock this stuff, they sell OEM spares themselves. Ask specifically for the "DECOY charger" or "battery charger for DECOY MK1/MK2" (your 2023 bike will be the newer MK2-era part).
• A couple of YT-authorised parts resellers overseas (e.g. Day Zero in NZ) list original YT DECOY chargers and cable harnesses as stock spares, which tells you it is a genuinely orderable OEM part - worth asking your nearest YT contact or a European reseller if the US side is slow to respond.
Worth flagging while you're waiting on that: some owners have found generic "universal" 42V chargers with a compatible physical adapter can work in a pinch, but I can't verify the pinout/communication protocol well enough to tell you that's safe for your BMS - I wouldn't risk a no-name unit on this one. Chase the genuine part through YT support first.