Hi, In the first sentence, "as" can be replaced with "because." So, "because" or "as" in this sentence indicates a transition in the sentence from "what" the speaker wants to "why" the speaker wants that particular outcome. He/she hopes that the people will come so that he/she can hear about the trip. In the second sentence, "because" indicates a causal transition from the first part to the second. Again, as in the first sentence, a reason is being provided as to why the dessert is all gone: "because" someone ate it all the night before. In the third sentence, "since" offers yet another transition indicating yet another reason. The items (whatever they are) are expensive because they are difficult to obtain. In sentences one and three, the words in question are adverbs, in sentence two, "because" is a conjunction.

Hello Talita! All of these are used to mean 'because' but they are used in slightly different ways. 'As' and 'since' usually introduce a reason that is already known by the person we're speaking to. They're not surprised when we give them this information¡ e.g. "I have to fire you Jon, as/since you've been late for work ten times this month". Jon KNOWS he has been late ten times. 'Because' also introduces an explanation, but it's more usually followed by new information. e.g. A: "Why aren't you coming to the event later Jon?" Jon: "I'm not going because my ex-girlfriend will be there". I hope this helps!