seperate tickets - booked one ways

hey everyone

my trip in august is tokyo-osaka-singapore-beijing-seoul-tokyo. these are all booked as one way flights on different airlines because it was cheaper that way. i added beijing to this trip thinking i could use the 72 hour visa but now i am not so sure.

i enter from singapore on sunday and leave for seoul on tuesday, so hours isn't the issue. again, these are seperate tickets.

think i'll be okay to use the 72 hr free transit? thanks!