동포(同胞): 동 same 포 placenta. Brothers and sisters born from a single
parent, friendly expression of people from the same country or people
of the same nationality.
In practice, It refers to most overseas Koreans. Especially, use for in china or in russia. Because, in my opinion, it is an old word, and usually used before 1950. And in that time, koreans usually visit china(중국동포) or russia(especially ukraine. Soviet government force moved koreans to ukraine. 우크라이나 고려인 동포 ukraine korean 동포).
교포(僑胞): 교 people residing abroad 포 placenta. A person who has settled
in another country and lives as a citizen of that country.
In practice, It refers to Koreans who are in japan(재일교포) or in USA (재미교포) or in south america (남미교포). Because, in my opinion, the word was usually used after 1960s. And in that time, koreans usually visit japan or amreica for get job.
교민(僑民): 교 people residing abroad 민 people. Foreign students living in
other countries, temporary residents, etc.
In practice, in my opinion, it refers to Koreans who are not a large group. Like 과테말라(Guatemala) 현지교민. Not using for koreans who living in america. (재미교민 X, 재미교포 O)
Details (korean): 교민 교포가 아니라 동포