There seems to be a bit of confusion in the community as to what the practical uses are for the third nametable supported by MMC5 (in EXRAM mode 0). The example mirroring setups using 3 nametables on the wiki (both on the "Mirroring" page and the MMC5 page) are frankly of dubious practicality. There was also a thread on romhacking.net a while back asking whether it was possible on hardware and/or emulators to use four-screen mirroring with MMC5, the thread creator evidently being under the impression that this would be far more useful.
Don't think of "3-screen mirroring", think of "vertical mirroring and one-screen mirroring at the same time, with all the advantages of both". MMC5's third nametable allows you to have unlimited 4-way scrolling with a status bar, no visible attribute clashes on any edge of the screen, and no need for complex raster IRQ voodoo ala Crystalis.
Here's how you do it: You put your playfield on the two built-in nametables, configured with vertical mirroring (MMC5 $5105 = $44) In your IRQ handler, switch to one-screen mirroring with the EXRAM nametable ($5105 = $AA), where your status bar resides. It's as simple as one-screen mirroring, but without the need for fine-grained playfield updates and without the unavoidable attribute clashes. Because your playfield is vertically mirrored the horizontal scroll seam can be well offscreen, and the vertical scroll seam can be hidden behind the status bar.
Moreover, since you're only using a small part of the EXRAM nametable for your status bar, you can even use the rest of EXRAM as work RAM, just as long as you don't need it while the status bar is being rendered. For example, assuming your status bar is at the bottom of the screen, you can put your music engine variables in EXRAM. Just flip the EXRAM mode to 2 at the start of your NMI music update routine and flip it back to 0 at the end.
Don't think of "3-screen mirroring", think of "vertical mirroring and one-screen mirroring at the same time, with all the advantages of both". MMC5's third nametable allows you to have unlimited 4-way scrolling with a status bar, no visible attribute clashes on any edge of the screen, and no need for complex raster IRQ voodoo ala Crystalis.
Here's how you do it: You put your playfield on the two built-in nametables, configured with vertical mirroring (MMC5 $5105 = $44) In your IRQ handler, switch to one-screen mirroring with the EXRAM nametable ($5105 = $AA), where your status bar resides. It's as simple as one-screen mirroring, but without the need for fine-grained playfield updates and without the unavoidable attribute clashes. Because your playfield is vertically mirrored the horizontal scroll seam can be well offscreen, and the vertical scroll seam can be hidden behind the status bar.
Moreover, since you're only using a small part of the EXRAM nametable for your status bar, you can even use the rest of EXRAM as work RAM, just as long as you don't need it while the status bar is being rendered. For example, assuming your status bar is at the bottom of the screen, you can put your music engine variables in EXRAM. Just flip the EXRAM mode to 2 at the start of your NMI music update routine and flip it back to 0 at the end.