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You won't find any 321s on the line to Cambridge, as @dk1 rightly points out, your best bet is on the line from London Liverpool Street to Ipswich. Stratford and London Liverpool Street are your best bet, sadly for you that will be the closest they get to Cambridge.
Bethnal Green is a good start (gets you everything else on the Great Eastern Line), but it won't get you all the 317s as it will miss any on Stratford -> Bishop Stortford working
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