Ok, so you did not get the large bias for i=3. Strange.
I got large bias for i=3 and theta > 1 even with a million outcomes, but for theta < 1 (I have tested on theta = 0.1 and 0.3) it showed quite good results.
I am attaching patch v3. Among other things I fixed small typo in description of random_exponential function in pgbench.sgml file.
— Thanks and Regards, Alik Khilazhev Postgres Professional: http://www.postgrespro.com The Russian Postgres Company