Nothing.

They might have permission to.

If they sell the same items on all 3 accounts and you have proof it is the same seller, report them to seller performance.

do nothing and worry about your business.

Unless the three accounts are selling the exact same items then you cannot do anything.

You could try minding your own business.

I love how many people have told you to mind your own business, we have 5 amazon accounts but 1 is for each company

I see no reason to have 2 for the same products but hey you never know

That is for professional selling accounts, you can sell as an individual on the others, EIN is not required

Typically they make you “Apply” To have a second account but will say no everytime so just do it

Its against the code of conduct for a seller to own multiple accounts selling the same item/offering same services regardless of FBA or FBM. You can artificially manipulate listings by using multiple accounts and sabotage the marketplace real estate. We face this often in our ink/toner category and deal with BOT THOUGHT MAYHEM but usually have to have an account manager specifically help us report so that we dont get crap templates that don’t address the issue… Sadly, unless you pay for some privileges in AMZ, its likely you won’t have a strong case manager to help move this along. If you try to address it to AMZ as hindering customer experience or negatively impacting buyer experience, it may get some ooompf!

Here is the official AMZ policy

https://sellercentral.amazon.com/gp/help/help.html?itemID=200386250

Multiple seller accounts:

Operating and maintaining multiple Seller Central accounts is prohibited. If you have a legitimate business need for a second account, you can apply for an exception to this policy: Go to Contact us.

Click Selling on Amazon, then select Your account, and select Other account issues.

In your request, please provide an explanation of the legitimate business need for a second account. To be considered for an approval:

You must have a separate bank account for each Seller Central account. We will not approve multiple Seller Central accounts that use the same bank account within the same region. If you sell across regions (for example in North America and Europe), you can use the same bank account for your Seller Central accounts as long as your accounts are linked though Amazon Global Selling.

Each account must have a separate email address.

The products and services sold in each account must be different.

Your Performance Metrics must be in good standing.

We will respond to your request within 2 to 3 business days. Failure to comply with these requirements might result in your account termination.

Thanks. I will look into that.