A. Tuscany/Chianti area (6 nights): Sept 7-13
Of the 3 locations (A,B, C) this one is the one that is best located to visit most famous towns and cities: Florence, Chianti, San Gimignano, Volterra, Monteriggioni, Certaldo, Colle Val D'Elsa. This is actually the closest of the 3 if you want to visit Pisa and Lucca. Siena is equidistant (in terms of time) from A and B, but in miles A is closer as well.
Therefore you should probably allocate the most nights to this location A.
B. Tuscany/Val d'Orcia area (6 nights): Sept 13-19.
This area is good for the Val D'Orcia, the province of Arezzo (Arezzo, Cortona), Castiglion del Lago in Umbria, and even all the way to Orvieto. Even Perugia is close by. Siena is probably closer from Greve (location A). This area deserves your second highest allocations.
C. Umbria/Montefalco area (6 nights): Sept 19-25
This area is good for most of Umbria, but probably 6 nights are excessive. All you have left to visit is Assisi, Gubbio, Spello, Spoleto, Todi, and possibly Perugia, unless you visit from the Val D'Orcia. It doesn't take long to visit those towns, they are really small.
I would divide the nights something like this, give or take, depending on what you want to visit and for how long:
A: 7, B: 6, C: 5
If you want to see Florence for 2 days and Siena for 1 from Greve you might want to allocate even more to Greve.
Of the 3 locations Greve is closer to the most important large cities and famous sights.