I believe the fact that the vast majority of stands at T2 will be jetbridge stands, and the layout, means that Flybe will remain in T3. Don't forget, it's proposed that T2/T3 will be linked airside, regardless of how long the walk is means it's less important for Flybe to be in T2.
I don't think T3 would be big enough for a Flybe/Ryanair and Easyjet scenario.