If you have 3,000 lbs. distributed over 12 square feet, that gives you 250 lbs. per square foot. That's a lot. You will need to check with local building codes to see what the requirement is (because it's likely that's what your house was designed to handle). Age of the house and specific blueprints would also be good things to have handy, just in case anything in your house is "non-standard". Any local architect would be able to give you a referral to a structural engineer; they have to work together to get just about all housing designs approved.
Personally, I keep all my tanks on the ground level, on a concrete slab. Better safe than looking at a 12 square foot hole in the floor.
Remember the immortal words of Ogden Nash:
"Some primal termite knocked on wood,
and tasted it, and found it good.
And that is why your cousin May
fell through the parlor floor today."
or the even more immortal words of Ogden Gnash:
"The sschnurg beast is a clever chap,
he's got some brains, he ain't no sap.
Then why is it he ain't so great?
Surely we don't underrate
his latent talent to create.
That is proved beyond debate.
The reason for his deep-sunk rut,
is that he won't get off his b-tt."
Sorry, it's been a long day.