Fail to make the printings fit to each other.

[Roberts_Clif]

Oh I remembered something, when loaded the first two parts, FlashPrint told me:"Errors has occured to the model. You can" and the options: "Repair model", "Delete model", "Ignore".And I pressed "Repair model".Perhaps it does something to the dimensions.

I loaded both models with several version of Cura no errors posted.

Your flashforge adventurer 3 has a 150mm Build area you have plenty of room.