Innanzitutto, benvenuto! Mi fa piacere vedere qualche Italiano qui!
As far as your quest for a lifetime Breguet watch goes, I am afraid that I am not familiar with a Ref. 3347 perpetual. I checked with a friend from Breguet as well, who, off the cuff, could not place it either. Are you sure that this is the correct Ref. number?
In general terms, I find that Tourbillon watches, for obvious reasons, are much more emblematic of the Breguet watches than perpetuals. Among the two you mention, I actually prefer by far the Ref. 3657; I have had it in my hands in both RG and PT, and it's an absolutely stunning watch. At 40mm, it is not too big, and I have seen friends with relatively small wrists wear it with ease.
Personally, I do not like the 3757 as much. Mostly, this is due to the two openings on the sides of the Tourbillon cage which show the screws of the tourbillon bridge. I never found Breguet's tourbillon bridge particularly attractive, and I really cannot understand why they would go to such lengths to show it. But that is just my own very personal opinion, of course!
If money were no object, my choice would go for the 3657, which I find one of the best looking tourbillons available on the market today.
However, I am also very curious about the perpetual calendar you mention, so if you are able to get more information on it, and perhaps a description of its dial layout, I would be very interested in reading it.