I think "optimal" is a matter of personal preference, space, and utility (probably in that order). First is a question of how much you want to look at. Is it just for an "ultimate" build, or are you looking at a beautiful system. Then, how much space do you have? You'd be surprised, but I've seen people spend money on three monitors and not have the desk space for them. Might want to consider VESA mounting it possible.
Finally, what are you needing these for? Gaming? Productivity? Design? If it's just productivity then three 27" is probably overkill. If it's gaming, then it'd be important to find out if your game can even support that kind of resolution, but then you're also probably looking at response time. If it's design, you're probably looking more at the color accuracy than anything else.
Almost forgot budget. Never forget budget.
All in all, three monitors 27" might be a little much, but if you can get it, why not? I imagine 23" is probably more reasonable.