Here is an example:
Assuming your table structure looks something like this:
Make sure you are enforcing referential integrity
Go to the "MainTable" and edit it's After Insert data macro
Make it's code look something like this:
This says that after a new record is inserted into the MainTable it will create a record in TableTwo and define the SerialNumber field, and the same for TableThree. If there are any other required fields for these tables you will need to set them here as well.
Here is the code from the pic above. You can copy and paste this into the Macro editor window and change names as needed.
Code:
<?xml version="1.0" encoding="UTF-16" standalone="no"?>
<DataMacros xmlns="http://schemas.microsoft.com/office/accessservices/2009/11/application"><DataMacro Event="AfterInsert"><Statements><CreateRecord><Data><Reference>TableTwo</Reference></Data><Statements><Action Name="SetField"><Argument Name="Field">TableTwo.SerialNumberFK</Argument><Argument Name="Value">[MainTable].[SerialNumberID]</Argument></Action></Statements></CreateRecord><CreateRecord><Data><Reference>TableThree</Reference></Data><Statements><Action Name="SetField"><Argument Name="Field">TableThree.SerialNumberFK</Argument><Argument Name="Value">[MainTable].[SerialNumberID]</Argument></Action></Statements></CreateRecord></Statements></DataMacro></DataMacros>