In this study the researchers compared the energy expenditure of 1 set of resistance training with 3 sets of the same training (Heden T, et al. 2011).
The participants who were overweight young adults performed 10 exercises with 10 maximum repetitions. The resting energy expenditure and respiratory exchange ratio were measured at the start and at 24, 48, and 72 hours after the exercises.
All of the participants performed both protocols separated by 7 days.
One set of resistance exercises which took 15 minutes to finish was as effective as three sets of the same exercises for elevating resting energy expenditure for 72 hours after the exercises.
If you are going to do resistance exercises to help you lose weight or avoid gaining weight, you can save time by only doing one set.
Heden T, Lox C, Rose P, Reid S, Kirk EP. One-set resistance training elevates energy expenditure for 72 h similar to three sets. Eur J Appl Physiol. 2011 Mar;111(3):477-84.