Generally, 3-RM is 91-93% of 1-RM.

Baechle, T.R., Earle, R.W. and Wathan, D. (2008) Resistance Training. In: Essentials of Strength Training and Conditioning. Ed: Baechle, T. R., Earle, R. W 3rd edition. Champaign, IL: Human Kinetics.

Thank you so much sir.

dear Sajith Udayanga

1 Rep Max Percentages

Reps %1RM

1 100

2 95

3 93

4 90

5 87

6 85

7 83

8 80

9 77

10 75

11 73

12 70

and You do not need an equation that calculates site easily without equation

http://www.bodybuilding.com/fun/other7.htm

dear Sajith Udayanga

Use Brzycki equation (Brzycki, 1995)  [1RM = W x 36/ (37- R)] where the W= weight (Kg); R= number of repetitions .

Brzycki, M. (1995). A practical approach to strength training: Masters Press Indianapolis, IN.

What Prof Warren gave you is still the best reference where you can find the table REPS vs. 1RM% .

I just attached a very brief table which pretty much summarise the same. 

M

http://www.elitefts.com/wp/wp-content/uploads/2013/11/elitefts-mladen-exertion-level-2.jpg

Mladen's table is very good! But i'm using Essentials of Strength Training and Conditioning Table 15.7. i think this is best table.

Thank you so much for everyone.

The Beachle,Earle and Wathan reference is very commonly used however most methods give a similar outcome.