Gold is a precious metal and has monetary value. It represented Jesus' Kingship.
Myrrh is a resin (dried sap) from the Myrrh tree. It is used in perfumes and incense. It was commonly used as an embalming ointment in Jesus' day. This was a startling gift for a young boy--it represented His death.
Frankincense is another resin, an aromatic one, from the milky sap of the Boswellia Tree. it is used as perfume, incense and in aromatherapy today, and was used in religious rites in Jesus' time. This represented homage to God.
All of these were costly gifts, and worth their weight in gold.
Frankincense and Myrrh were used for perfume, medicine, and embalming the dead. Clumps of incense were discovered among the treasures buried in the tomb of Tutankhamen. They were considered the most THE MOST valuable treasures a merchant could posses (at the time of Christ), and were worth their weight in gold.