The three characters
已 are visually very similar, but do they have a common etymology or any overlap in terms of semantic content?
Any information would be appreciated.
「已」(/*ɢ(r)əʔ/, already) was originally the same character as「巳」, and is also a phonetic loan. Much later on, the two meanings (sixth earthly branch and already) were differentiated by writing the character「已」without a fully enclosed loop at the top.
「己」is graphically unrelated to the other two, and the original meaning is unclear. The earliest determined usage is for the word /*k(r)əʔ/, sixth heavenly stem. The meaning self is a phonetic loan.