Arena events do not show on the apps currently. They are newer formats and haven't been added.
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Chess.com mobile app do not show all tournaments (only a few). For example, regular 3+2 arenas doestn show as 'Online Tournaments' in the app. But if i select 'Daily tournaments', i see web-page from chess.com (if i understood correctly) with different tournaments, includes 3+2 etc. So it should be?