A Type 3 LSA represents an advertisement of a network from another area.
Area Border Routers (ABRs) do not forward Type 1 or 2 LSAs between areas.
If a ABR receives a Type 1 LSA from a area, it creates a Type 3 LSA referencing the network from the Type 1 LSA. The Type 2 LSA is used to determine the network mask of any multi-access networks. The ABR advertises the resulting Type 3 LSA into other areas.
If an ABR receives a Type 3 LSA from area 0, it recreates a new Type 3 LSA for the other area, listing itself as the advertising router with an additional metric cost.