(UPDATE) 3:53PM JST Yomiuri: The pressure of the containment vessel in the Reactor No.3 has "stabilized"; for now, no releasing of the valve is scheduled.
Yomiuri Shinbun (in Japanese; 1:44PM JST 3/20/2011):
TEPCO has announced that they will try to lower the pressure of the containment vessel in the Reactor No.3. The pressure, which was at 2.8 atmospheric pressure at 1:10AM on March 20, rose to 3.4 atmospheric pressure by 4:30AM. TEPCO will stop the work at the plant to restore power, and stop dousing while they try to lower the pressure.
The valve to release steam from the containment vessel in the Reactor No.3 has been open since 8:41AM on March 13. It is not known why the pressure is rising again. TEPCO will try to open this valve again if it is closed. If they cannot open the valve, they will try to open the other valve. There is a possibility that highly radioactive substance may be released if they have to open the other valve, as it will be released without going through coolant (water).