What is the symbolism of the brooch, a cup, and a knife in the barrow?

The brooch, the cup, and the knife are the Sleer's most precious possessions. He suggests that he is magically bound to the sole purpose of their protection and has been unable to leave the barrow since the disappearance of his master. The three items, just like their guardian, collectively symbolize the insatiable spirit of greed. Greed inevitably leads to the self-destruction of anyone who feeds it for long.
The objects' symbolism becomes most apparent when Bod tricks the greedy and conniving Jack Frost into trying to tame the Sleer. Jack Frost enters the barrow, with a captive Scarlett, and makes a plan to dispatch the Sleer and take his treasure. He thinks he can exploit the Sleer's desire for a master to control and escape from him. As soon as Jack Frost declares himself master, the Sleer recognizes his avarice and coils around him, absorbing him into the wall.