NATIONAL Australia Bank is expected to be followed by at least one of its rivals in directly tapping shareholders for funds to strengthen their balance sheets, after it successfully raised $3 billion of new capital yesterday - $1 billion more than it originally thought possible. In a sign that a floor is beginning to be put under the recent falls in financial markets, NAB surprised even itself when investors snapped up an initial offering priced at $20 a share and pushed the underwriters of the issue for an additional amount of 50 per cent.