Meta, Apple, and Alphabet, which owns Google, are being looked into for potential breaches of the Digital Markets Act (DMA) introduced in 2022.
If they are found to have broken the rules, the firms can face huge fines of up to 10% of their annual turnover.
EU antitrust boss Margrethe Vestager and industry head Thierry Breton announced the investigations on Monday.
Just six companies have obligations under the DMA, but they are also the world's largest tech firms: Alphabet, Apple, Meta, Amazon, Microsoft and ByteDance.