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Collagen Type III is the second most abundant collagen in the body. It is closely related to collagen type I in terms of location and manner of synthesis. It is primarily located in the intestinal walls, muscles, and blood vessels, although it can also be found with collagen type I. Collagen type I and type III can be mixed together to create supplements for the skin, bone, and muscles. Type-III collagen is a fibrous scleroprotein^[1] in bone, cartilage, dentin, tendon, bone marrow stroma and otherconnective tissue; yields gelatin on boiling.
Collagen Type III has been has been listed at the US National Library of Medicine Medical Subject Headings (MeSH).^[2]

^[1] Scleroprotein is a simple protein found in horny and cartilaginous tissues and in the lens of the eye.
^[2] https://www.nlm.nih.gov/cgi/mesh/2011/MB_cgi?mode=&term=Collagen+type+III