Will these alternators all be on the same shaft operating synchronously? If not, "minor" differences in output power will bring havoc to any interconnection. Eventually you'll want to hook them to a MPPT converter (to maximize performance) and a load dump (to prevent overspin/overcharge). If you treat each alternator individually, three controllers could each be smaller and redundant.
[edit] I should also comment that if these are all on the same shaft, and are nearly identical alternators, then you should be able to wire them in series as Alternating Current sources. Yes, with one diode bridge around the whole mess. You can easily align the second and third rings so that the total series voltage is maximized. Then regulate battery charging (and overspin) with a buck regulator and a load dump (if necessary). [/edit]