I can't tell you the "Bootstrap way" to fix this, since I don't use it. I can tell you the why: Your content doesn't allow it at that breakpoint.
Because rows of tables don't wrap around (ie. all of the cells for each row must appear on the same line), there is an absolute minimum width that every given table can resize down to, which depends on things like paddings and the widest non-wrapping content for each cell. For your tables, the smallest they can go is whatever the width of "Column", "Column", and "10/11/2012" equal out to plus 48px (8px padding on each side times 3).
The span4
s are only allowed to be 31.49% of 724px (definition on .container
ancestor element), which isn't wide enough to contain the tables at their smallest possible width.
My recommendation would be to not try and place all 3 tables side by side. Your display would have to be exceptionally wide in order for it to not look cramped and not have horizontal scrolling.