The word jealous is polysemous; it has several glosses enumerated in a good dictionary like the American Heritage : Envious of : All

1. things very funny that one of my coworkers got promoted.
2. What happened to her boyfriend in a relationship where she was jealous of her old male friends?

What is there to be learned from this?

1. I am jealous of our gods name. I am jealous, but I am not jealous of this.

…

For the most part, your last scenario corresponds to #1 here in being a matter of simple Envy (the only one of the seven abyss that is no fun at all). When, in your first scenario, you say that A is jealous, A may be jealous of C in sense #2 (as illustrated in the American Heritage example) or of B in sense #4.

If you ask a couple who have an over-friendliness towards C whether A love B or C then at this stage you ask whether you are jealous of B or C or both? It’s different, and it’s mostly ambiguous. Natural language is more intelligent and more resilient. How do I get used to this?

And speaking of getting used to it, note how describing the scenario becomes easier and more (ahem, straightforward) without that anxious death-grip on hetero-normativity.