www.orchid.systems/.../inter-entity-trade

You mean, that by default, Sage 300 doesn't have this option. Is it correct?

Could you review my question?

Creating additional databases is of course no issue.  These will in no way be child entities in any way other than how you code the flow of transactions in your programming.

Connecting to SAMLTD and reading transactions - no problem
Connecting to AR1, AR2 and AR3 to write transaction read from SAMLTD - no problem

But, by default, can I create a few Subsidiaries myself in Sage 300?
Or do you mean that for each subsidiary we should ask Sage 300 support to create a separate DB?

I want to understand if can I create a few Subsidiaries myself in Sage 300 and then via API to receive Invoices via these Subsidiaries

There are options (as indicated).  You would be well advised to review with your Sage Business Partner.

Natively, in Sage300, there is one GL and one AR and one AP, etc, per company.  You can use department codes in the GL to segment your account and have a separate RE account.  In AR you can use AR account sets to segment them if you like but they are apart of one AR ledger.

But if you're looking for a second or third subledger separate from the first - not possible within the one entity.