I've mounted my existing dual-monitor setup on my Ergotron. Everything pretty much went as expected.
The tipping hazard mentioned by others is real, but is not necessarily a given. If you have relatively small side monitors, the unit won't tip over. Put monsters on it and all you need are a few books to brace the unit while you mount monitor #3.
Because I have mismatched monitors, the bezels do not perfectly line up. Part of the problem is the depth (thickness) of each monitor is different. If you have a thicker monitor in one position, the bezel will be closer to you than the bezel of the neighboring monitor.
This didn't bother me - I kind of expected it - and like most other reviewers don't even notice it when using the system. If you're a perfectionist with unrealistic expectations, trust me - you'll get over the "problem" in no time.
The spring-assist mechanisms in the unit are very strong, and were set to maximum from the factory. My unit had nearly 30 full turns of adjustment, but the system never became loose or slack.
With the height and tilt settings adjusted to minimum, the unit was still essentially non-adjustable with only one 22" widescreen and one 19" standard monitor fitted. No matter what, it would rise to the uppermost position and highest tilt - there just isn't enough weight with two semi-small monitors to make it behave!
I literally had to tie the arm down to keep it in place. I hope a third monitor will make the difference; otherwise I'm in trouble.
After fitting the 22" and 19", it is evident that the stand can handle significantly wider screens (as per the specs). My 22" diagonal widescreen is 20-1/8" wide, and the 5:4 standard is 15-7/8". With them fitted, the arm has another 2.5" of linear travel available before the monitor mount won't work.
So, to calculate if your monitors will fit add the actual width of the center monitor and one side monitor, and divide by two. If it is 20.5" or less, you're OK. In my case, (20-1/8" + 15-7/8") / 2 = 18, leaving me with 2.5" of margin.
Because of the geometry, thickness doesn't play a major part in this, but if you're right at the edge with three identical monitors (that will line up nearly perfectly) you might want to leave yourself a bit of extra margin. Personally I'd say 1/8" is enough.
The screens do rotate, but if you want portrait mode you'll need to keep the arm height tall enough to keep the rotated LCD from hitting your desk. (Seems duh, I know, but you never know.)
Damaging the rear foot may have been a blessing in disguise, as I ended up removing it completely. I have a desktop shelf (hutch) behind the stand on my desk, and the rear foot of the Ergotron ended up hitting the post of the shelf unit before I got it into the required position.
I could have moved the post, but the shelf may have become unstable. Removing the Ergotron rear support gained me two much-needed inches of depth.
On removal, it is evident that the rear foot does serve a purpose. With the monitor stand near or at the tallest position, it is possible for the stand to tip backwards ever so slightly. The rear foot normally prevents this. So don't remove it unless you have some kind of other support (like a shelf or wall) behind the Ergotron.
(Ergotron would undoubtedly frown on this - the rear foot is screwed and glued, and they obviously want it there. So do this entirely at your own risk. You can re-attach the foot by re-bolting it on with a little epoxy if it doesn't work out.)
The problem diminishes once the monitors are on the stand, because the LCD weight forces the stand to lean forward on to the big front foot. But if the arm is set tall it might still happen.
The stand is set pretty tall, and the monitor height ended up being higher than the height I had my desk stands set to. That may be just me; I'm sure I'll get used to the new setup. Maybe it'll keep me from slouching quite as much.
In the end, everything lined up quite nicely and tightly - I wouldn't be able to get the monitors any further away without hitting equipment behind them. I have only two monitors fitted because my new triple-monitor workstation is not here yet, but I'm looking forward to the upgrade.