Please Chapter 12 Is So Massive Already - Tumblr Posts

How does one post a main chapter to Ao3 then link an offshoot of that chapter?

Chapter 12 has three parts and aaaaaaa there's 12, 12.A, and 12.B

12.A and 12.B are optional reads and whatnot but aaaaaa I wanna post them all together but I'm struggling to understand Hooooooowwww ;A;