They are not all taken (LLL's). They once were.However in september 2018 the co-regstry have changed a policy.
It is so that the new policy is as follows : the registry charges from then on mostly 1.250 USD or 1.500 USD (mostly) and (for a certain amount (this is only for some : not enormously much ones) of LLL .CO's 3.500 USD or more for SOME special LLL.CO's (not all though) LLL-domains that drop AND BECOME AVAILALBLE to REGISTER. HOWEVER FOR THESE DOMAINS renewal (or transfer to someone who buys the domain from somebody) is ONLY the NORMAL COST, so the normal renewalprice for a.co-domain (which is between 20 USD/year-35 USD depending from registrar to registrar). It is so that lots of people AS CONSEQUENCE of that new policy, and also because of high DEMAND for LLL.CO's if the corresponding LLL.COM is occupied or people ask 50.000- 500.000 for the LLL.COM, DON'T let their LLL.co-domains drop, but renew them at normal pricing. (letting them drop gives the risk that once the domain comes available again, immediately costs 1.250 USD - 3.500 USD registryfee for the 1st year new year, so people who have LLL.CO's don't drop them easily for the moment.
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Another thing changed also at 1 september 2018 with the LLLL-domains that drop : The price in case of drop for such a domain; changes to around 125 USD (instead of low pricing), but renewal afterwards costs normal renewalprice.(20-35 USD)
For 5L-domains registrations is as you see at registrars or a promotionprice; renewal is again between 20-35 USD depending from registrar to registrar.