Here's my personal opinion.... you're 18 right? You are still trying to find your own identity.... most 18 year old boys want to have a 3 some.... they're at the peak of their "lifetime hornyness" and many 18 year old guys are looking to have sex with a hot girl... as much as they can get in any given period. Generally the first thing NOT on their mind is "I want to marry this girl and settle down".... if you have a 3-some you are redefining your relationship with him.... he will no longer see you as you are but as this "kinky wild fun girlfriend"... which is FINE if you don't want the relationship to last but don't expect this situation to make you closer or think he will have more repsect for you. Now if a couple who is well established and older wants to experiment in the bedroom.... then yes I'd say THEY should go for it because they are adults who have dated for a long time and have a better grasp on what works in a relationship and what doesnt.... plus an aged female in a long term adult relationship is less likely to do something for her husband that she doesn't want to do simply to "keep him around".
If you love this guy and want to have a long term relationship with him where you see your name linked to his in marriage one day then NO
If you want to live a fun carefree 18 year old life and don't care if you're with him in a year or two AND are curious yourself then YES
If you don't want to do it and are simply doing it to avoid him breaking up with you then NO and.... learn some self preservation.
See the thing is he has told me that this fantasy of his is something he would want if he was single and he doesn't really wanna do it with me since we are in a relationship and being in this relationship with me that's not even on his mind and not something he really wants to do should I take that into consideration that he doesn't wanna do it with me since we are in a relationship?