The three main mid-ocean ridges aligned NS are separated by about 90° along equator. The division line of upper mantle directly above lower mantle division is according to analysis a downwelling one, so ocean ridges do not tend to appear exactly there. A good example is the Reykjanes Ridge, following the path 1.5° to the west of the main division of lower mantle. Therefore, the three ridges are as close to being 90° apart along equator as the system allows.
This is of course the theoretic distribution. The Mid-Atlantic Ridge is magnificent. But it is under the ocean like the other ridges.
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