Hi Florencia,
It depends what you are buying the piano for, what condition the piano is in, whether you expect to restore it or not. When you say a newer Bechstein, what do you mean by newer? Do you mean pre - WW2 1930s era? Postwar? Gaveau can be decent pianos but again it depends on the condition and age.
Also, we've no idea what size the pianos are that you are looking at. It would help to know.
It's hugely likely that the three pianos you mention here, if they are in original condition, are at the end of their useful musical life. When I say hugely likely, I also realise that there are some pianos that travel through time and retain their sound and structure exceptionally well over the years.
I have found in the UK that a lot of the un-restored Blüthners and Bechsteins from the period 1900 to 1930 are displaying tell tale signs of dead soundboards (weak treble, in some cases to the point of virtually no sound, and a booming bass), and the tuners who work on them speak of spongy tuning pins, a lot of rust on the strings, and actions that require a lot of rebuilding work. This is absolutely fine and normal, since we're talking about instruments that are 85 to 115 years old.
I have two aliquot Blüthners, 6'3, one from 1894 and one from 1912, the older one has been fully rebuilt and the younger one is in the queue for rebuilding. The older one has received a new soundboard, re-capped bridges, a new tuning plank, a new action, a new keyboard, re-strung, and refinished. The younger one will receive all the same work except I've chosen to retain the original paten action and keyboard on it. The price of this work is high of course, but fairly modest in comparison with buying a new Yamaha.
But not every old piano needs this kind of work. Firstly it depends on the condition of the piano and secondly it depends on what the piano will be used for. Some pianos can be tuned and regulated and sound fairly decent even after 100 years, and OK they're not going to be in gleaming new condition anymore but they're perfectly functional. Some might need a new set of hammer heads, or restrung and the original plank and board may well be serviceable enough to merit a restring without replacing them (personally I don't recommend this, but I know it's done, and sometimes it works out OK).
Consider the advice I've just given you as a bit of a 'worst case scenario' for the pianos you are trying out. An Internet forum can be a good place to gather information but ultimately it takes an experienced technician to actually see the pianos in question before you will be able to make a choice. You see, although it's quite normal for a piano of this age to require a full rebuild, an experienced technician may sit down at it, play it, test the tuning, and say that the piano will be good for another ten years or so in its current condition.
The other very important factor is, do you actually like the pianos in question? Forget the names, the models, the makes, for a second. Do you enjoy the sound and touch of these pianos? Which one of them do you like the most?
