The report was written by RSMC Beijing.
RSMC stands for "Regional Specialized Meteorological Center", and there are 8 of them around the globe. The RSMCs operate under the World Meteorological Organization (WMO), and are "prepared at all times to provide highly specialized computer-based simulations that predict the long-range movement of air-borne radioactivity". (For more, see the page on RSMCs at WMO.)
The particular report, titled "Summary Report of RSMC Beijing on Fukushima Nuclear Accident Emergency Response" (CBS/CG-NERA/Doc. 5.4 ), which you can download here (Word file), was presented at CBS Expert Team on Nuclear Emergency Response Activities in Vienna, Austria, held from October 31 to November 4, 2011. In it, on page 8, RSMC Beijing says:
On 15 March, an explosion was heard in Unit 2 and damaged the pressure-suppression system, causing the leaks of radioactive cooling water. Shortly afterward, Unit 4 was damaged by an explosion and a large amount of radioactive materials was released into the atmosphere. At 11:00 (Japan Standard Time) JST on 15 March, Unit 3 explored again. At that time, due to the easterly winds and precipitation in and around Fukushima, the surrounding areas including Tokyo, Nagano, Sendai and other places detected high radiation, which matched well with the simulation results.
In the text, "explored" is clearly "exploded". TEPCO lists only one explosive event for Reactor 3, and that's on March 14 at 11:01AM. Where did RSMC Beijing get that data that Reactor 3 exploded twice? Looking at the pages 2 and 10 of the report, the request from IAEA for data on the Fukushima accident went out to RSMC Obninsk (Russia), RSMC Tokyo (Japan) and RSMC in other Asian Countries but RSMC Beijing has been in charge of compiling joint statements.
From Page 2:
In 2011, RSMC Beijing for EER is the chief RSMC in RAII, which is in charge of organizing the emergency response activities among RSMC Beijing, RSMC Tokyo and RSMC Obninsk and composition of the joint statements of RAII.
In other words, the mention of the 2nd explosion in Reactor 3 on March 15 must have been made after consulting with RSMC Tokyo. (EER stands for "Environmental Emergency Response"; RAII refers to Asia.)
It sure looks there are a whole lot of things that the Japanese government and TEPCO haven't been telling the rest of us lesser folks.