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"over three centuries" would not mean "in excess of three centuries" unless it came in an expression like "they lived there FOR over three centuries". This is the history of a mansion over three centuries - i.e. a period spanning three different centuries (say C16th to C18th) OR 300 years, though it now seems pretty clear it is the latter.
Can you confirm that the period "los trescientos años investigados", in your new context, corresponds exactly to the period referred to in your question: "Propietarios e inquilinos en tres siglos"?
From the material you provide, I'd don't see the slightest problem with either of the two answers provided. I assume that mediamatrix was just going out of his way to make sure that we weren't all making a foolish mistake. All's well that ends well.
Please read this review. It clearly says "los trescientos años investigados", so why complicate things unnecessarily?
En este segundo título, ‘Historia social del Palacio Oneto’, destacan las sagas familiares de los Coig, Oneto, Albreda y Osborne, de las que se aportan interesantes datos biográficos de sus componentes más significados e ilustres, entre los varios centenares de moradores de esta Casa Palacio, durante los trescientos años investigados.
Broca, a way to be at peace about this is to count the hundreds of years the book covers (or almost covers). If it doesn't come close to 300 years, either anser could be adjusted. If it's 268 years, for example, it could be "over more that two centuries" or "for almost three centuries." As I have no specifics dates, I couldn't advise you if this is necessary.
The 'point' is that both answers (so far) imply that the period in question was (at least) 300 years - not, as you confirm, that the period may have been much shorter.
Jenni and Pablo are confusing the 'instant' with the 'period'.
Asker hasn't given much context, but if this is advertising blurb for a UK audience, for example, the proposed translations may land someone in hot water.
"Over three centuries" means, in the most generally-accepted sense of the expression, "in excess of three centuries".
"Across three centuries" is slightly more ambiguous - but Pablo's webref confirms his own misinterpretation.
I don't see what point you're making at all. "Over three centuries" could also mean a period that began, say, some time in the C18th and finished some time in the C20th. It would still be "over three centuries". Why is it so important whether it means that or 3 x 100 years? The asker clearly needs to know which preposition to use.
... we follow the logic of the two answers so far, and broca's own interpretation, we must all be at least 200 years old since we were all born 'en' 19XX and are still living today, 'en' 2011.
The only reasonable, logical assumption you can make from the ST as quoted here, is that the period in question was 100 years plus 2 days; for example: 31st December 1799 to 1st January 1900 (inclusive).
That's well short of 'over/across three centuries', which implies a period of approximately 300 years.
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Answers
3 mins confidence: peer agreement (net): +5
Over Three Centuries
Explanation: My personal Choice, but you could also phrase it "Three centuries of Owners and Residents" or something like that.
-------------------------------------------------- Note added at 3 mins (2011-11-16 21:49:19 GMT) --------------------------------------------------
My personal "choice" with a lower case c!
Jenni Lukac Local time: 04:29 Specializes in field Native speaker of: English PRO pts in category: 68
The translation workplace 
peer agreement (net): +5