3 C refers to Class III, subcategory 'C'
there used to be 'A' 'B' 'C' 'D' and 'S' which were 1 litre, 1.3 litre, 2 litre, 4 litre and 6 litre
I think there was a 3 litre class at some point, at least in europe
Adam will be able to give better details than me.
Hope this helps
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