...the clutch arms...
Of course, the NIG clutch (HSA536) in the SRF3 doesn't really have 'arms'.
Because of that I think the most likely cause is the spring...
Part number HSA128. Same part, old design and NIG, BTW.
I would replace the spring no matter what, as you can't tell for sure from inspection if it is weak.
Back in the day, your LBS would have one of these in the 'Sturmey parts drawer'. Thanks to the internet, you can still get most any
part you need for Sturmey.