GRP = Glass Reinforced Plastic
FRP = Fiber Reinforced Plastic
BMC = Bulk Molding Compound

These are all terms for generally the same thing. The glass reinforcing in GRP typically comes in fibers, but not all fibers are glass. Aramid (Kevlar) or carbon are examples of other reinforcing fibers used in similar applications, though I don’t know if these are used for standard manhole covers. Bulk Molding Compound is a thermoset plastic that is impregnated and reinforced with fibers. So in essence, the first two terms describe the reinforcing, and the last term describes the material that is being reinforced.
FRP manhole cover
Telecom manhole cover
So I can’t be absolutely certain that all of the above are the exact same for a given application, but generally, I believe they are referring to the same idea. A cast iron manhole cover is an example of a cover that doesn’t fit into any of the above categories.
manhole cover