Turn up the heat and do the plastic ASAP. Get some fans circulating the air as well. If you are growing mold, you need to get that shit under control!
Long term, make sure the windows are well-sealed, and look into either replacing the windows or getting storm windows. Since you are on the 3rd floor, interior storm windows might be a good choice.
I'm not sure how the overall insulation of the space plays into this, but also something to look into, especially with rooms that get extra cold.
re mold: YMMV, but alcohol or hydrogen peroxide might also be good choices for disinfecting (DO NOT MIX WITH BLEACH), but I'd do a little research first, depending on what material the mold is growing on. And either way, it doesn't matter until you've stopped the condensation in the first place.