Once completed, Khusab-3 will reportedly be a 50MW plutonium production reactor. Construction began in 2006, and open source satellite imagery suggests that the reactor may be completed sometime in 2011. [1]

While Khushab-1 is moderated by light water, Khushab-3 may be moderated by heavy water. If so, plutonium production could be achieved utilizing less power. Assuming Khushab-2 and Khushab-3 will have production capabilities similar to Khushab-1, as fuel from both reactors becomes available for reprocessing, Pakistan's annual production of weapons-grade plutonium could triple. [2] The facility is not under IAEA safeguards.

Sources:
[1] David Albright and Paul Brannan, "Pakistan Appears to be Building a Third Plutonium Production Reactor at Khushab Nuclear Site," Institute for Science and International Security, 21 June 2007, www.isis-online.org; Paul Brannan, "Construction of Third Heavy Water Reactor at Khushab Nuclear Site in Pakistan Progressing," 5 October 2010, www.isis-online.org.
[2] Global Fissile Material Report 2010, Chapter 10, International Panel on Fissile Materials, December 2010, www.fissilematerials.org.