I'm not the leading expert on this (Milkman?), but from the amounts of transistor-smoke I've generated over the years -
If an amp is rated at, say, 200 watts into 4 ohms, you'll get roughly half power (100W) at 8 ohms.
Most monitors have 2 jacks on the back, wired in parallel, so as you add speakers by plugging one speaker into the next, you are halving the ohmage.
(2 8-ohm monitors plugged one into the next = 4 ohm load to the amp)
When you bridge a stereo amp to mono, it requires twice the load as the 2 individual stereo channels did. ( minimum load for a stereo channel ( for this model amp, others are different) is 4 ohms, so the minimum load in bridged mode is 8 ohms.)
I have, however, read specs on some amps that say they'll work in bridged mode down to 2 ohms, so I guess it's not a hard-and-fast rule. However, all the specs I can find on Alesis amps say bridged mode has an 8-ohm minimum.
(If you run a solid-state amp into too small a load, you'll fry the main power transistors.)
After poking around the 'net some more, it looks like you'll be safe running 2 monitors off one side of the amp and one off the other. If the amp has separate volume controls for each channel, you should be able to crank up the channel with ONE monitor on it (it's 8 ohms, the channel with 2 monitors is 4 ohms) to compensate. Watch the clip lights.
I guess the bottom line is, wire it up to 3 monitors with the amp in stereo mode and see if it's loud enough. You won't damage the amp. I wouldn't run it in bridged mode into 3 monitors until you find out for sure if the amp can take it.