The Airbus does not set takeoff trim automatically, we have to set it. The value we enter in the Takeoff performance page is only there as a reminder.
TO trim setting is achieved with FMGC input ZFWCG/ZFW.
Not exactly correct as that is the ZFW trim setting, not the TOCG.
The TOCG we have calculated by either using the Loadsheet (if you're in the dark ages) or the LPC is indeed entered like you say on the Takeoff Performance page, and in your example if you wanted 2.6DN and a Flap 3 takeoff you would put 3/2.6DN in the FMGC.
As I said though this is only a reminder, so after engine start we would set 2.6DN using the trim wheel.