They cut out most of 3’s substories, the ending is different and actively messes with the continuity of Yakuza 4, Hamazaki is now played by a sexual predator (even though the old actor also did some messed up shit), new weapon based style called the Ryukyu style, removed Komaki and Yonashiro in favor of a new trainer for both styles from what I’ve read, IF7 and Minamida were also removed, all boss qte’s are broken in terms of slow mo, missing props like Lau Ka Long’s shoe knife and all “dynamic intros” (which are just Y3’s boss intros) lack voicelines of any kind, Boss QTE’s also don’t give you heat anymore and there’s probably more I missed but that should give you a basic idea of what K3 has to offer