Hmm... From what I can tell, other than them all having Tsuyopon in the lead and all being tear-jerkers, there's little else in common.

According to drama Wiki, they're thematic though. 'Boku no Ikiru Michi' is about living life to the fullest, 'Boku to Kanojo to Kanojo no Ikiru Michi' is about family ties and 'Boku no Aruku Michi' is about purity.
I watched the first episode of 'Aruku Michi'... It reminds me quite a bit of 'Algernon no Hanataba'. >_<