Originally the factory fitted 1 piece exhaust AFAIK, these were rpleaced bu 3 piece designs, cheaper to make, distibute and fit given the tight space and the variation in fit. If the 3 piece does the job then fit it.
When you say 3 bolt plate (flange) do you mean the single pipe 3 bolt downpipe flange like a carb RRC or the twing pipe downpipe which also had 3 bolts, 2 behind and one ahead?