The fusion of hard and soft pomerons : 3-jet diffractive production.

We consider the central exclusive production of high E_T jets pp\rightarrow p+(X+\mathrm{dijet})+p. In particular we study the possible contamination of the purely exclusive signal by semi-exclusive production where no other secondaries are emitted in one hemisphere, between the highest E_T dijet and the recoil proton, while in the other hemisphere a third jet, plus possibly additional hadron activity, is allowed, but still separated from the incoming proton by a large rapidity gap. This process arising from the fusion of a hard and a soft Pomeron has not been considered before. It turns out that it gives a negligible contribution. The calculation involves a careful treatment of the QCD colour structure of the amplitudes