Does shear strength increase with the number of fasteners?
If you have a single screw with that level of shear strength, it will also have minimum distance specifications for the material in which it is used. As an example, a fastener of 12 mm diameter should be placed no closer than 6 mm from the edge of the material.

Regarding spacing relative to one another, the material used is a factor as well as many other characteristics such as fastener diameter and material.

I found an interesting brief in PDF form here:

fastener spacing which opens directly into the PDF or a download window. It references spacing specific to wood trusses and fasteners and also takes into consideration wood grain.

There's an astonishing calculator located fastener calculator which uses a formula far beyond my comprehension:





In the PDF link, it is suggested for the reader to enlist the services of an engineer. If you are using loads of one to two thousand pounds, that might be a good idea.

On the other hand, if you are building something without an inherent catastrophe in case of failure, build it and learn. I wouldn't construct a bridge across which I would drive the family vehicle, but I might consider a bridge across a gully for a golf cart or similar transport.

Generally, if you have the spacing for three bolts, it will be stronger than one, if there is no slop or play or similar poor construction. A single bolt in a tight hole with two others in loose or misaligned holes will not be optimum and could fail prematurely.