Heat on the steel and cast iron should do the trick.
Aluminum is a bit tougher. If the steel is seized in the aluminum, the aluminum threads will likely become damaged by the harder steel when trying to back out the fitting. Also, be careful using too much heat on aluminum too quickly; it can easily warp and/or crack the aluminum. You can try to freeze the steel fitting by turning a can of air upside-down so it comes out as a liquid, but be very careful to avoid freezing your fingers, then quickly try removing the fitting before the aluminum gets too cold.
An odd trick I've used in the past is to hold an engraver against the fitting; the vibration sometimes was enough to crumble the corrosion between the dissimilar metals.
Lastly, repeatedly use a good penetrant and wait a few hours between applications and then try slightly tightening the fitting first before attempting to back it out.
Good luck
Ray