3 hours of ProRes HQ @ 1080/23.98 should be about 250GB so maybe you have your media doubled up? If you’d set your conversion to prores 4444 it would be able 360GB plus the original files i’m guessing around 120gb… would add up to half a terabyte? In anycase 500GB for a 3hour project is within the realm of reason.
In terms of long term storage. Hard drive space is cheap, time is valuable. Buy an external hard drive and backup the whole thing.