Proposed by Robin Carr et al. from Astex Technologies [1], Rule-of-Three (Ro3) compliance is designed for fragment-based drug discovery [2, 3] and enforces the following properties:

* Molecular weight ≤ 300;
* Octanol/water partition coefficient, ClogP ≤ 3;
* number of H-bond donors and acceptors less than, or equal to 3;
* three or less flexible bonds;polar surface area ≤ 60 Å2.

References:
[1] Congreve, M., Carr, R., Murray, C. and Jhoti, H. A ‘Rule of Three’ for fragment-based lead discovery? Drug Discov. Today 2003, 8, 876-877.
[2] Fattori D: Molecular recognition: The fragment approach in lead generation. Drug Discov. Today 2004, 9, 229-238.
[3] Hann MM, Oprea TI. Pursuing the leadlikeness concept in pharmaceutical research. Curr. Opin. Chem. Biol. 2004, 8, 255-263.