The blackbody emission power is independent of the environmental temperature. The 3 K in space is irrelevant to the amount of power that the metal will emit. If you're referring to the radiation that the metal would have otherwise absorbed in a warm environment, keep in mind that the Stefan-Boltzmann law goes as T^4. The amount of radiation lost by the really hot metal will be far more than the energy is absorbs from the surrounding environment, unless the environment is close in temperature to the metal.
More importantly, who says that molten metal is a good blackbody? The principle of detailed balance tells us that if a surface doesn't absorb much radiation, it can't emit much radiation. In terms of blackbody emission, that translates to an emissivity that changes depending upon reflectivity. Polished copper has an emissivity of 0.03 at 1.6 microns, meaning it will emit 3% of the amount radiation that a blackbody would at the same temperature (@ 1.6 microns). Molten iron has an emissivity of 0.35 @ 1 micron (red hot).
If you want to keep your molten metal hot in space, put it into a shiny box (oven) for it to cool. I just did a quick calculation for a 50 cm dia sphere of 1900 K molten iron in deep space. It would take 104 hours to cool to room temperature if it was floating in free space (although it would drop to 1000 K after the first 4 hours). If you put it into a polished platinum box (emissivity of 0.02), it would take weeks to cool off.