Harbin: two or three days is enough. If you come in winter, you can spend more time at the ice and snow sculptures, ice lantern. Central Street, Sophia Church, hunting ground and so on.
Tianjin: 2 day is ok. One day to see the sites in Tianjin City and one day to take a cuise ship to the sea.
Beijing has D trains to Harbin, 8 hours for 385/462RMB
Harbin have one D train to Tianjin: 8.5 hours, 381/457RMB
Ordinary trains are much cheaper, but spend much time. These bullet trains are cheaper than flights.
Hope it helps.