Based on THG Best Graphics Cards for the Money, June 2010
, there is no way that 3x 8400 will come close to a 260. 3x 8800s might work, but it's probably not cost-efficient.
Typically speaking, a generation old SLI config can keep up with current-generation or be a little faster. But the 8400s are now 3 generations old and the 8400 is pretty near the low end of the line.
My typical recommendation is that if you're 2-3 generations behind, just get the best new graphics card you can afford. If you're only 1 generation behind and your graphics are starting to slip, you could either SLI/CrossFire or buy a new graphics card, depending on the resale value of your current card and personal preference.