'First sentence...  *&%$#79686 585 ... Second sentence 89^&^    continues...      The third one..'

 'First sentence. Second sentence continues. The third one.'

 cat test
'First sentence… *&%$#79686 585 … Second sentence 89^&^ continues… The third one..'

 # sed 's/…/\. /g' test | grep -Po "(^\')?([a-zA-Z]+(\.\s?|\s)?|(\'$)){1,}" | tr -d '\n'
'First sentence. Second sentence continues. The third one.'

 line = re.findall('(^\')?([a-zA-Z]+(\.\s?|\s)?|(\'$)){1,}', line)

abc *&%$#79686 585 … 123 ... abc ... ...

 'abc. abc.'

 >>> s = 'First sentence...  *&%$#79686 585 ... Second sentence 89^&^    continues...      The third one..'
>>> import re
>>> s = re.sub('[^a-zA-Z. ]','',s)
>>> l = [i.strip() for i in s.split('.') if i.strip()]
>>> l
['First sentence', 'Second sentence     continues', 'The third one']
>>> l = [re.sub('[ ]+',' ',i) for i in l]
>>> l
['First sentence', 'Second sentence continues', 'The third one']
>>> '. '.join(l)+'.'
'First sentence. Second sentence continues. The third one.'
>>>