You should be able to get them by simply researching the Trade technology. Not sure if there are any other requirements for the core freighter design, but if you're three years in you should already have any there are (and if not, you should have the trade module to design your own with).
IIRC there was an issue in older versions where if you got Trade from an AI instead of researching it yourself, the Freighter and Trade Module wouldn't become available. So if you're still running the version right off the retail discs, you'll want to update to the latest release patch (version 1.2, available in the downloads section of the site here). Note that you'll need to begin a new game to gain the full effect of the new patch so you may want to finish your exisitng game first.
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