Most of the evidence indicates that doing more sets of the same exercises is a waste of time if you want to gain strength, but not everyone agrees.
Three sets are better than a single set:
Kelly, S. B., Brown, L. E., Coburn, J. W., Zinder, S. M., Gardner, L. M., & Nguyen, D. (2007). Effect of single vs. multiple sets for strength. ACSM Annual Meeting New Orleans, Presentation Number, 2507.
One set is better than three sets:
Fincher, G. E. (2000). The effect of high intensity resistance training on peak upper and lower body power among collegiate football players. Medicine and Science in Sports and Exercise, 32(5), Supplement abstract 657.
Fincher, G. E. (2001). The effect of high intensity resistance training on sustained anaerobic power output among collegiate football players. Medicine and Science in Sports and Exercise, 33(5), Supplement abstract 756.
One is as good as three sets (and takes less time):
Baker, J. S., & Cooper, S. M. (2004). Strength and body composition: single versus triple set resistance training programs. Medicine and Science in Sports and Exercise, 36(5), Supplement abstract 394
Hass, C. J., Garzarella, L, de Hoyos, D., & Pollock, M. L. (2000). Single versus multiple sets in long-term recreational weightlifters. Medicine and Science in Sports and Exercise, 32, 235-242
Thompson, J. A., & Terbizan, D. J. (2007). A comparison of single to multiple-set resistance training programs in males. ACSM Annual Meeting New Orleans, Presentation Number, 1785.
Starkey, D. B., Welsch, M. A., Pollock, M. L., Graves, J. E., Brechue, W. F., & Ishida, Y. (1994). Equivalent improvement in strength following high intensity, low and high volume training. Medicine and Science in Sports and Exercise, 26(5), Supplement abstract 651.
Starkey, D. B., Pollock, M. L., Ishida, Y., Welsch, M. A., Brechue, W. F., Grames, J. E., & Feignbaum, M. S. (1996). Effect of resistance training volume on strength and muscle thickness. Medicine and Science in Sports and Exercise, 28, 1311-1320.
Hass, C. J., Garzaarella, L., De Hoyos, D. V., & Pollock, M. L. (1998). Effects of training volume on strength and endurance in experienced resistance trained adults. Medicine and Science in Sports and Exercise, 30(5), Supplement abstract 651.
Wolfe, B. L., Vaerio, T. A, Strohecker, K., & Szmedra, L. (2001). Effect of single versus multiple-set resistance training on muscular strength. Medicine and Science in Sports and Exercise, 33(5), Supplement abstract 435.
Wolfe, B. L., LeMura, L. M., Razzaghi, M., & Szmedra, L. (2002). A quantitative analysis of single vs multiple sets in resistance training. Medicine and Science in Sports and Exercise, 34(5), Supplement abstract 1123.
Gomes, P. S., de Paula, A. M., Diogo, C. E., de Freitas, M., Rodrigues, F., & Pereira, M. I. (2003). Effects of single and multiple sets resistance training on strength gains of previously experienced adults. Medicine and Science in Sports and Exercise, 35(5), Supplement abstract 1621.