I have seen にします being used in sentences which I believe to be depicting desire. Is it interchangeable with が欲しいです or たいです, or both? Doumo arigatou gozaimasu ^^
-ni shimasu means "I'll have (the pie, a sandwich, a coke)" when ordering at a restaurant or choosing from some options that someone has given you.
hoshii means I want (something)
-tai means I want (to do) ...
The short answer is no. Can you give us some examples of sentences that confuse you, because that would be easier to explain than just the abstract differences among the three.