A Canadian randomised controlled trial compared nebulised 3% saline with 0.9% saline in 81 infants under 2 years of age with bronchiolitis. The short-term use of nebulised 3% saline did not result in any statistically significant benefits, although a non-significant trend toward a decrease in hospital admission and improvement in respiratory distress was found. A larger study would be required to determine whether these trends arise from a clinically relevant treatment effect.

There’s really not much that’s been shown to make a difference in this disease, as this review article reminds us.
Effect of inhaled hypertonic saline on hospital admission rate in children with viral bronchiolitis: a randomized trial.
CJEM. 2010 Nov;12(6):477-84