First, to read your list into an array, one entry per line:
readarray -t countries
…or, with older versions of bash:
# same, but compatible with bash 3.x; || is to avoid non-zero exit status.
IFS=$'\n' read -r -d '' countries || (( ${#countries[@]} ))
Second, to duplicate the entries, either expand the array to itself three times:
countries=( "${countries[@]}" "${countries[@]}" "${countries[@]}" )
…or use the modern syntax for performing an append:
countries+=( "${countries[@]}" "${countries[@]}" )