One of the goalies should be considered as trade bait. It won't be Niemi, as he's by far the Sharks number one goalie.
Many people believe that Niittymaki should be traded, but I don't think he should. Teams may want to avoid him because of his injury problems. Despite the injuries, he's a reliable backup goaltender for the Sharks, if Todd McLellan uses him right.
However, Thomas Greiss is younger, and more valuable. The Sharks would most likely get a greater return for Greiss than Niittymaki, if any team even wanted to pick up Niittymaki.
Since the Sharks have three goalies, then by all means they should shop Greiss around. Such a trade would improve the team. The Sharks could use a penalty killer or a depth forward with 20 goal capability.
I'm pretty sure one of the goaltenders is going to be traded soon, perhaps around the deadline. The Sharks don't need to carry three net minders. As I said, the Sharks could get some great value back for Greiss.
Once Niittymaki returns to the crease for the Sharks, Greiss' days on the west coast could be numbered.