The regulatory question raised here—whether currency in a rotorcraft category carries over to fixed-wing privileges—touches on a foundational structure of 14 CFR Part 61 that trips up many pilots pursuing multiple category and class ratings. The answer is straightforward but easy to misunderstand: recency of experience requirements under 61.57 are category- and class-specific. Takeoffs and landings performed in a helicopter do not satisfy the three takeoffs and landings within the preceding 90 days required to carry passengers in an airplane single-engine land (ASEL) aircraft, and vice versa. Each category/class combination the pilot holds must independently meet its own passenger-carrying currency requirements. The one item that does transfer across all the pilot's ratings is the flight review requirement under 61.56 (assuming no other exception applies, such as completion of a WINGS phase or possessing an ATP certificate)—a single flight review, regardless of which category/class it's conducted in, satisfies the 24-calendar-month requirement for all certificates and ratings the pilot holds, since 61.56 is written as a certificate-level requirement rather than a category/class-level one. This is a critical distinction: passenger currency (61.57) is per category/class, while the flight review (61.56) is per certificate.
For working pilots, this matters most acutely in scenarios like the one described: a professional helicopter pilot who flies for a living but maintains a private airplane certificate on the side. Because 61.57 currency doesn't transfer, that pilot could be fully "current" and proficient in the Bell or Robinson they fly daily for work, yet be legally prohibited from taking a friend up in a Cessna 172 without first logging three takeoffs and landings solo (or with an instructor, as sole manipulator) in an airplane within the preceding 90 days. This is a common trap for pilots who assume that because they fly constantly for their job, they're "current" in any airplane they're rated in. It's also a reminder that currency and proficiency are not the same thing—a pilot may be current under the regulation while being genuinely rusty in a category/class they haven't flown in months, which is itself a safety consideration independent of legal compliance.
This question also reflects a broader pattern seen across corporate, charter, and GA flying: pilots increasingly hold ratings across multiple categories—airplane, rotorcraft, glider, or even powered-lift—whether for career flexibility, personal enjoyment, or because EMS, tour, and utility operators value multi-category pilots. Part 135 and Part 91 operators who employ pilots with mixed certificates need to track currency separately for each category/class an employee might exercise privileges in, particularly if that pilot occasionally ferries or demonstrates aircraft outside their primary duty aircraft. Flight departments and chief pilots should build recordkeeping systems that flag 61.57 currency independently per category/class rather than relying on a single "currency" checkbox, since regulatory violations here are easy to commit inadvertently and carry real enforcement risk if a ramp check or accident investigation reveals a pilot carried passengers without meeting class-specific currency.
Finally, the original poster's understanding of 61.57(a)(1) is correct as it applies to day, non-tailwheel, non-complex passenger-carrying operations in ASEL: three takeoffs and landings to a full stop or otherwise within 90 days, as sole manipulator of the controls, plus the biennial flight review. Where pilots often get further tripped up is with night currency (61.57(b)), which requires additional takeoffs and landings to a full stop at night within the preceding 90 days if carrying passengers after dark, and is again category/class specific. As dual-rated pilots become more common in both the professional and personal flying communities, understanding these nuances of 61.56 versus 61.57 remains essential not just for FAA compliance but for genuine risk management when transitioning between dissimilar aircraft types.