FRP combines the best of plastic and fibreglass and tries to reduce the disadvantages of both. Also, from experience, plastic is harder to paint properly (and make it stick) whereas fibreglass is easier to work with. Fibreglass is also the cheapest of the 3, FRP and plastic is almost the same price. Note that there are also varying degrees of quality of bodykits, like some of the fibreglass kits we have seen are really thin and poor quality, whereas some are much more sturdier and won't feel as if they'll fall apart when shaking them.
Below is taken from Wikipedia:
Glass-reinforced plastic (GRP), is a composite material or fiber-reinforced plastic made of a plastic reinforced by fine fibers made of glass. Like graphite-reinforced plastic, the composite material is commonly referred to by the name of its reinforcing fibers (fiberglass). The plastic is thermosetting, most often polyester or vinylester, but other plastics, like epoxy (GRE), are also used. The glass is mostly in the form of chopped strand mat (CSM), but woven fabrics are also used.
As with many other composite materials (such as reinforced concrete), the two materials act together, each overcoming the deficits of the other. Whereas the plastic resins are strong in compressive loading and relatively weak in tensile strength, the glass fibers are very strong in tension but have no strength against compression. By combining the two materials together, GRP becomes a material that resists well both compressive and tensile forces. The two materials may be used uniformly or the glass may be specifically placed in those portions of the structure that will be experience tensile loads.