In the Oxford English Dictionary I found the following words where a letter occurs three times in succession.
A list was posted on Quora and all I've done is check which ones the OED lists, plus I thought of oooh.. So there could be others not discovered. Apart from brrr, oooh and yayyy my favourite is frillless, which makes perfect sense, although you don't hear it very often.
brrr
duchessship
frillless
hostessship
oooh
uuula (I guess this is 'uvula' spelt before u and v were regarded as different letters)
vertuuus (which means more or less the same as 'virtuous')
yayyy (!)
Other -ship words are hyphenated, e.g. countess-ship, and I think the OED is just being inconsistent with the two examples above.