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Those lucky enough to grab one of the three home run balls Shohei Ohtani belted last Friday night might be needing some bigger wallets soon ... 'cause one prominent auction expert tells TMZ Sports they could each be worth millions!

David Kohler , founder of SCP Auctions , said on Monday if he had one of the historic Ohtani mementos on his block ... he would anticipate it selling for "$3 million or more."

It's a staggering number, but Kohler said it's not far-fetched at all ... considering the famous 50/50 home run ball Ohtani smacked last year ultimately sold for $4.39 million .

In fact, Kohler told us he wouldn't be surprised if one of the balls from the big National League Championship Series clincher fetched even more than that.

"The market is prime

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