For the 3-ring benzos, the 7 position has to have a strongly electron-withdrawing effect. The '2 position has to be small so that the ring can sit at an specific rotational position compared to benzodiazepine ring. Banzos have been hugely studied and the QSAR is complete. I cannot recall the name, but I bought a paper that went through several 100 different 3 & 4 ring compounds. It's the most addictive class of the lot.