Alec: Jace
Jace is always flirting with you but you both know it doesn't mean anything and he just do it for fun or to annoy Alec. But Alec sometimes thinks you may like Jace and not him anymore. But you know that's impossible because you love Alec with all your heart and you could never do that to him.
Jace: Magnus
Because you are a warlock he thinks that with Magnus your relationship will be easier and you could be always with him. Even tho Magnus is sometimes a little flirty with you you know that Jace is the only person you love and have eyes for and whenever you have the chance you remind that to him.
Magnus: Simon
You know Simon for about 15 years and you are very close to him. Every time Magnus is busy you call Simon to come over your house because you are bored. You have talked about this with Magnus and he had told you about his jealousy but your assured him that Simon is your best friend and you see him only like a brother. Magnus trust and believe you but he continues to not trust Simon. But only if he knew how much you love him and only him...
Simon: Alec
Even if Alec doesn't want to get into downworlder business he is a little more friendly towards you but ONLY friendly and not something more. But Simon doesn't understand that and he is always pissed and jealous when Alec is around you or close to you but you assure him every time that Alec is just a friend. And besides he has Magnus.