I really don't know it but... at least I can guess the meaning of ㄷㄷㄷ.
Here are some tips...
ㅎㅎㅎ means 하하하 [hahaha] "joyful laughter"
ㅋㅋㅋ means 크크크 [keu-keu-keu] "sneaky laughter"
ㄷㄷㄷ means ...???
ㄷㄷㄷ maans... [deu-deu-deu] "hesitating..." (don't know it exactly)....
Is there anyone who knows about ㄷㄷㄷ?