Help with HDDs

So I have 3 HDDs gathering dust. I want to make a NAS from those however the old PC I have (Which I plan to use for this) only has 1 HDD connector internally. I have seen splitters online and want to purchase them and also power connections. Is this a good way to go about it? Also the HDDs will not have a place to be mounted inside the case as it has a mount for only 1. I have seen some HDD cases which let you mount a lot of HDDs are those good for this? are they stand alone and do not need me to use my old pc? Any tips for this to work?