NO, is the short answer @AlanJolly65.

Three UK and iD are independent mobile network operators.
The only thing they have in common is that both use the Three UK mobile network infrastructure. 

Devices are locked to the mobile network operator - anyway, good luck @AlanJolly65. 

Thank you for a clear response. Much appreciated 

I received my 3 network locked device today and happy to report that it works with IDMobile and Smarty SIM cards.

This is in a 3 Network branded and locked router. 

Maybe other devices vary but I'm happy this router allows any SIM card that can connect to the 3 network

Hello @AlanJolly65,

The short answer as @andewhite said is No.

That said it may work (which you reported it does) because of the use of the same network but we just wouldn’t never guarantee it.

I’m pleased it’s all working fine.

Mohammed

Thank you for clarifying Mohammed.

I totally accept that it will only work with some locked devices (luckily mine is one of them)

Therefore I would say that there is no short answer. As the answer in "maybe"

Are you sure the Three MiFi router is network locked?

Three UK haven’t been selling network locked devices for several years now.

Anyway, good to know it’s working for you @AlanJolly65.

That's a good point.

I havent tried but It is branded as ""3", the product description says it's network locked and many of the reviews (Amazon) confirm it's locked although others report success with "Smarty" SIM cards

My advice would be to only buy a device where you can return it if it doesn't work.

Fair enough @AlanJolly65 - not too surprised it works with a SMARTY SIM card, as they’re Three UK’s in-house MVNO brand. 

Out of interest, are these pre-owned or new Three branded MiFi routers?