A 3-5g tank will not be large enough for a fry tank IMHO. You need to keep the water VERY clean, and fry need room to grow. Plus small tanks are less stable, temperature swings faster, ammonia builds up very fast, etc...
The fry will likely just die without proper heat and filtration.
I would just keep males then you don't have to worry about having fry.
10-30g would be better. I don't know what size your main tank is, but I recommend at least 30g for mollies as they can get 3 to 5 inches and are very active swimmers.
Used tanks or DIY containers are cheaper. You might be able to set up a 10g as a Sump to the main, that way they can share the heater, but with the house that cold, you may still end up needing a second heater, depending on how strong your current heater is.
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