hey
The basic rule with all PA sound systems is that the amps should be 1.5 to 2 times more powerful than the RMS/AES power of the speakers. Initially this does seem a bit strange to some people, who question it and ask well if your amp is capable of putting say 500w into a speaker and its only rated for 250w, arent you gonna blow it?
Well, basically the answer is almost certainly no your not. While it is theoretically possible, the power raiting of the speaker will have been measured using a sine wave or pink noise, something with constant power. Real life music isn’t constant it has loud and quiet bits both at the microscopic, split second level and over the course of a song, and so while it may have short peaks that are fairly high (that the speaker can handle no problem) the average power is much lower.
However if you look at the reverse argument and have an amp that is the same or lower rated than the speaker, chances are your gonna have to push it very hard, as loud as it will go, to get the volume your looking for and to drive the speakers properly. Running the amp at its limits like this, and probably clipping and overloading it, will cause severe distortion in the music signal. As well as making your music sound frankly shit this will build up a huge amount of heat in your drivers magnet/motor system and be MUCH more likley to blow it. So going against what most people would initially think, using a smaller amp is actually far more likley to blow and damage things than using a big one.
General rule is an amp that is 1.5 to 2 times bigger than what your speaker is rated at, and run it about 75%-80% so that it isnt being pushed to its limits and has plenty of room for large transients and peaks and delivers a nice clean, undistorted signal to the speakers.
Now with all that being said, lets look at your exact situation.
PLX amps are fairly decent pro amps and are rated for 2ohm loads. With a sound system, as you start adding speakers together (in parallel) the impedence (measured in ohms) goes down. Most speakers start at 8 ohms, so double this to two speakers and the impedence halves to 4 ohms, and if you double the speakers again to 4 then impedence halves again to 2 ohms. Being in the middle and having three 8 ohm speakers give you an impedence of 2.666 ohms.
So in theory you could indeed take your first amplifier and connect three speakers in parallel to each channel (left and right) of one amp. The speakers would be showing a 2.6666 ohm load on each channel and be rated for 750w RMS, and at this impedence the the amp would be able to deliver probably about 700w.
This would be underpowering all your speakers but as long as you were sensible with the power and you were getting the volume you needed without pushing the amp to its max and into clipping/distorsion, it’d be fine. You’d then have the second amp to run the monitor speakers, which would be 125w with an 8 ohm load and the amp would deliver 300 watts so this’d be about perfect just again be sensible with the power levels and dont drive them stupidly hard and you’ll be awesome.
However long term you would really want to be looking for a bigger amp for your main speakers to really get the most out of them. An amp that is stable to 2 ohms still, but can deliver about 1500w at 2 ohms, would be much better for you. Sticking with the QSC PLX range to keep things the same then a 3002 would be about perfect for you.
However this all asumes that your speakers are all playing the same thing, which you havent said. If some of your speakers are subs and some tops, then you’ll need a crossover somewhere to split the music up and deliver the bass to the subs and the mid and trebble to the tops. This may be built into the speakers or may need to be a seperate bit of electronics in the rack with the amps, and you may also need further amps to balance things out and work all the different parts.
Hope this helps, and if you can give more detail on exactly what the speakers are and what brands/models etc i could help more 
k
