If you haven't been told specifically which accessories must be part of the ring..
Then nothing stopping you making sockets 1,2 & 3 the ring..
And just spur the FCU outlet off socket 3.. {a little less conductors to pull in}
(Appendix 15 page 555 gives an examples)
It can be argued that its easier to connect 3x each conductor LN&E into a double socket accessory.. with just a single LN&E feeding into the FCU.
Because the FCU will limit the load along the spur to 13A anyway there is no specific need to make it part of the ring itself.
Welcome to the forum..
And well done for asking a question and also giving your perspective of how you understand the problem as well..
(too many just come on here asking to be spoon fed answers! )