The quarters are based on the calendar year so Q3 is the current price list running from 1st July to 30th September. I believe it used to be the case that new cars / models would be added at the start of a quarter with little or no changes until the next but this has now changed with cars / models being added and removed part way through.
If your renewal is due this quarter its worth remembering that you can extend the current lease if below a certain mileage so if your heart is set on the T-Cross and it isn’t added this quarter it may appear on the next or one after, having extended you’d be able to order at some point in the future without penality or loss of good condition bonus.