Six narrow seats survived a cross-check of manufacturer specifications and independent fit guidance. The SlimFit3 LX combines a 16.7-inch shell, three modes, an explicit three-across design claim and an integrated belt lock-off; exact fit still depends on the car, seat positions and the other two restraints.
Purpose-designed for three across and the only shortlisted all-rounder with 24 source mentions in the focused scan. Its belt lock-off is useful when center-seat LATCH is unavailable.
A slim, upright shell with extended rear-facing capacity. Rigid LATCH can simplify an outboard install, although a seat-belt install may package better.
Only slightly wider, but ClickTight is a meaningful advantage in a cramped back row. It is a strong choice when secure installation matters more than the last half-inch.
Start with the Graco. Graco calls the SlimFit3 LX its slimmest design and explicitly says it fits three across; the listed limits cover rear-facing, forward-facing and booster use. gracobaby.com
Choose Clek for extended rear-facing. Both the Foonf and Fllo are 16.9 inches wide with 50-pound rear-facing and 65-pound forward-facing limits. clekinc.com
The infant-seat pick is the KeyFit 35. Its 16.5-inch listed width, European belt routing and LockSure system make it a practical narrow bucket, though the handle and neighboring seats must still clear. chiccousa.com
Diono needs a caveat. The Radian 3RXT is very narrow and spans RF, FF and booster modes, but Diono’s three-across statement is specifically for combinations of Radian seats. diono.com
| Seat | Type / role | Width | Limits, lb | Installation edge | Source |
|---|---|---|---|---|---|
| Graco SlimFit3 LX | 3-in-1 | 16.7″ | RF 40 · FF 65 · B 100 | Integrated belt lock-off | gracobaby.com |
| Clek Foonf | Convertible | 16.9″ | RF 50 · FF 65 | Rigid LATCH | clekinc.com |
| Clek Fllo | Convertible | 16.9″ | RF 50 · FF 65 | Rear-facing lock-offs | clekinc.com |
| Britax Poplar | Convertible | 17.0″ | RF 50 · FF 65 | ClickTight | britax.com |
| Diono Radian 3RXT | All-in-one | 16.5″ | RF 50 · FF 65 · B 120 | Slim shell | diono.com |
| Chicco KeyFit 35 | Infant | 16.5″ | RF 35 | LockSure + European routing | chiccousa.com |
Method: searched 1,804 unique web results across manufacturer, independent test and recommendation pages on 23 August 2026; the final shortlist cross-checks six model families. Width means published or independently measured external width, not guaranteed usable bench space. RF = rear-facing, FF = forward-facing harness, B = booster.