Help with Interval timer connected to 3 nios ii processors

Timers are so small I recommend using one for each CPU. That said it depends on how you use them whether sharing them would cause a problem. If you didn't use them as a timer in the BSP settings and you just took snapshots of them only from all three processors that should work although you might see some weird glitches if multiple processors attempted to take a snapshot at the same time. As soon as you use them as a timer in your BSP (system timer for example) then you would run into issues because you would have multiple timer drivers attempting to service the same interrupt which would be a nightmare.