Most wireless routers automatically bridge both the wireless and wired networks into one. All you need to do is set up the printer wherever it's connected as a shared printer. Open up Printers and Faxes, right-click on it, select Sharing, and then click the box for "Share this printer". Then on the other machines, just choose to add a networked printer through the "Add a printer" wizard. You'll be given a list of printers on the network (in your case there should only be one anyway), select that one and the rest is the same as if you were installing a locally-attached printer.
Alternatively you could buy a print server: that way, you wouldn't need to always keep the computer hosting the printer turned on.
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