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As it stands, all of your PCs are connected to the Network - but it sounds like File Sharing isn't enabled on one or more of the machines. The network is simply the connection between all of the PCs, but this doesn't provide the space or facility to share files - you need to pick which PC(s) you want to store files on.
Other PCs can only access those files when the PC you are sharing files from is turned on, so if you are just looking for a single place to store and share files, I'd pick the PC you use the most. The method for setting up a network share differs in each OS (easier the newer it is), so if you let us know which (or all?) OSes you would want to set up the network share on and we can find a tutorial

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If you want all of the PCs to access a shared network space no matter which computers are on, it may be better to buy a NAS (network attached storage). This is storage device that you plus in to your router/modem and all of the PCs on the network can see/use it - quite handy if you want to backup items or store photos/documents. There's the cost of the hardware (probably around $100-120 for a 1TB NAS drive), so it's not a cost-free option like setting up a network share on an existing PC... but I thought I should mention it just in case!