Tranche 1 & 2 are more than sufficient to replace the F3 fleet. The question of Tranche 3 was, and is, how comprehensive the GA capability was ever supposed to be and how much the partners would be willing to pay for it.
Cancelling Tranche 3 would almost certainly lead to the Jag force replacement being provided the JSF. That would require some additional airframes, but we're supposed to be buying 150 to replace a GR7/9 fleet of around 60, so I wouldn't imagine the numbers would be great.
That might lead to the Jag force being extended by another couple of years, but that would depend on the version chosen and the production rate. They could just thin the number per squadron across the fleet to transition both the GR7/9 and Jag fleets adding more later. I would imagine the bottlenecks would be factoring in the additional groundcrew/aircrew training into the present transition plan.
I don't believe the total joint orders of JSF and Typhoon were sustainable in view of the planned force structure. Cynically, which order was to be cut would reflect which would save on design and procurement costs and, importantly, which was seen as providing both a justification for a UK production line and more work for BAe and RR. The facts would seem to lean towards JSF.