Nota Bene: this solution doesn't answer exaactly to the question, it does too much relatively to the expressed need.
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In Python language:
import re
pat = '(?:(.)(?!.*?\\1.*?\\1.*?\\1.*\Z))+\Z'
regx = re.compile(pat)
for s in (':1*2-3=4@',
':1*1-3=4@5',
':1*1-1=4@5!6',
':1*1-1=1@',
':1*2-a=14#a~7&1{g}1'):
m = regx.match(s)
if m:
print m.group()
else:
print '--No match--'
result
:1*2-3=4@
:1*1-3=4@5
:1*1-1=4@5!6
--No match--
--No match--
It will give a lot of work to the regex motor because the principle of the pattern is that for each character of the string it runs through, it must verify that the current character isn't found three other times in the remaining sequence of characters that follow the current character.
But it works, apparently.