See the Wikipedia link I posted:
https://en.wikipedia.org/wiki/English-language_vowel_changes_before_historic_/r/#Mary–marry–merry_merger
"The full Mary–marry–merry merger (also known, in this context, as the three-way merger) is found throughout much of the United States (particularly the Western and Central United States) and in all of Canada except Montreal.[7] This is found in about 57% of American English speakers, according to a 2003 dialect survey.[5] The merger is highlighted in the song Merry Go 'Round, whose central wordplay revolves around "Mary", "marry", and "merry" having the exact same pronunciation in the singer's accent."
There's a map here (scroll down about halfway, or search for "Mary") : https://brilliantmaps.com/linguistic-maps-that-divide-americans/