1 is basically a prologue tbh. 1 and 2 where actually meant to be a single game when the game first started development but 1 was shipped by itself because it had to be as the dreamcast needed something to compete with the PS2. They do a lot to pad out 1 with the forced working at the harbor and waiting for X time on X days because of how small in scope it is to 2.
2 is definitely more vital to 3. 1 can basically be summed up as "Ryo's dad was killed, he wanted revenge and found a way to get to china to get it" where as there is loads of lore dumps in Shenmue 2 that can't be summed up so easily. The last 2-3 hours of Shenmue 2 are also you meeting Shenhua and learning about her and she is gonna be your main companion in 3 which starts in the cave right at the ending of 2.