Hi. We have noticed over the past few days that our 9 month old Goldie appears to have a more obvious and slightly enlarged 3rd eyelid that occasionally looks reddish. I was after any advice as to what may be causing this. Thanks in advance.

Sounds like inflammation.  I would want a vet to look at it, just in case antibiotics are needed.  May not be anything, but with eyes it pays to be overcautious.

I wd be wirried about the third eyelid showing vets wd be my option.