There are several places that even if separated into suites/apartments, offer great location for weddings AND will only take them on if you book the entire property so you have it exclusively.
First one that comes to mind is
Dianella - I've been there for a best friend's wedding and it was truly magical! It is now a wine resort and perfect for couples with no kids, taking advantage of the winery on site and the chef/restaurant on the premises if you wish.
Next one that comes to mind is
Villa di Ulignano - perfect size for 30 guests, it is pretty glamorous but at the same time would make the entire wedding super special.
Since you're interested in July, which is HIGH season, I recommend you contact them ASAP to find out about availability and cost, if you already have dates make sure to specify which 3 nights. Also if you want them to offer catering and any other services, most of these have partners who they work with if you aren't working with a planner.
Another one that just came to mind is Villa di
Liliano, which isn't too far out of Florence - but they are divided into apartments rather than rooms, so that would mean couples sharing apartments rather than suites/rooms as the first two above. Take a look at these and let me know if you need more ideas!