spirilla (spiral) bacilla (rod) and cocci (spherical) :) happy i could answer this one!
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I'm going to give the nonscientific answer, a bacteria can only maintain one shape because it don't have enough cells to mold into another shape there for it is not multicellular (or it is I think it depends on if its arche or EU bacteria).
A 4 sided shape has 4 vertices whereas a 3 sided shape has 3 vertices.
3-D Shape
Wich 3 D shape has a corner on it?
A triangle as no other 2d shape can have 3 sides.