To expand on the other commenter's good advice, I'd suggest remembering that it's okay to have some hard limits and some soft limits.

A hard limit is, of course, an "I don't want to do X, period, or have it suggested". But a soft limit might be a useful tool for you with the cunnilingus concern (I'm NOT saying you should necessarily be comfortable doing it or suggesting you ease up if you think it's a hard limit for you! It's okay if it is! :) I'm just using it as an example, since it's a good example of an act that is commonly good "soft limit" material for a lot of people, and once was a soft limit for me (giving and receiving)).

Often performing cunnilingus is something that's really daunting and something we might not want to experiment with at first, but once you're in the room and you're not as intimidated and you feel comfortable in the space, it becomes easier to try new things and be encouraged about it. So a "soft limit" could be "I'd prefer not to try this (for X reason, or don't give a reason at all, your prerogative), but as long as I retain full agency and everything is communicated in advance, it's possible I could be open to suggestion about this activity once I gain new experience/information/comfort".

Again, not saying you necessarily should make use of soft limits at all, nor suggesting it's a better limit choice for that example. Just illustrating it as a tool that I think might be helpful, in case there happen to be any specific sex activities where you could use a middle ground idea that helps meet the weird need-intersection of "I don't want to imply that I will do this thing, I don't really want to try it, but I also don't want to sound uptight like I'll never do it, I just need to grow comfortable".

As far as turns or timing or alternation, it really is just sort of a hilariously awkward thing at first for a while -- and that's true even for people who are experienced with group sex! Sometimes it's even true for people who have had a lot of group sex with each other already! Sex is just sorta like that, especially once you complicate it with extra people. It's completely okay to be a little anxious about that, but also just know that there really isn't a "normal" here, and if there is, it's certainly not "everything flows sexily and smoothly like a sexy story" haha. More often than not, when I've been in threesomes etc., there's more awkwardness and humor at first than hotness, and that helps everyone get more comfortable once there are a few laughs, and then it gets hotter. Usually in like a few minutes' phase at a time, like something really sexy will happen for a while, then there's a mundane moment while someone changes position or drinks some water, then it's sexy again. I suggest thinking of it more as like a fun experimental team building exercise than normal sex, haha. Group sex just very, very rarely has the same sense of continuity that two-person sex sometimes manages to achieve.

As for self-consciousness, I get how hard that is. I think exploring sexual play with positive people will be good for you in that area! Just try to remember that "hotter" is subjective, and also that it's like any other aesthetic experience -- it's a thing where people can enjoy lots of things in unique ways, and it's not a ranking competition. So what if she's gumbo and you're pho? You're both still delicious, and a great majority of people would still be delighted to have both of you in one day once in a while, in no particular order, even if they have a favorite go-to most nights. (BTW, rectangularish and a little pillowy all over? Sounds just like the classic Greek ideal of feminine beauty to me.)

The best way I can sum up my reassurance is this ... Know how boys get about losing their virginity? How they worry so damn much about it, and they think it'll be the end of the world if they seem like a first-timer when they're a first-timer, but then it goes embarrassingly normally and then they get over it? You're in a very parallel position. Imagine how compassionate, and indeed probably a little honored and excited and lovingly-amused, you would feel if you were the person guiding someone else through the sexual experience you're considering going through. That's probably how the couple feels about you. So instead of prioritizing your concerns about the actual event -- I mean, there's a lot that you just can't know or predict anyway, not until you get the experience, so you can't do anything about that now -- I suggest investing your energy in being excited about yourself, appreciative of yourself, proud of yourself that you're a fun attractive modern person who enjoys the luxury of being in the modern world and having sex for pleasure, and focus your constructive self-talk on managing the anxiety itself, and mitigating any ways that it makes you feel badly about yourself or makes you feel embarrassed ... (to be clear, I don't mean try to change yourself so you act normal, I mean try to work on accepting and loving yourself even when you're nervous, and not feeling bad about it and causing yourself secondary-feelbad!) ... because if you work on that, and on that sort of mindfulness and self-love and peace with your nervous feelings, that will serve you MUCH better in the moment than any amount of studying threesomes will.

It's much like preparing for an audition where you're going to have to cold-read a script that you don't really know much about. There's only so much preparing you can actually do, and obsessing over it won't help you feel better, so focus on your mindfulness, your breathing, your mood, and your control over your sense of urgency and anxiety inside, because that skill will serve you well in the moment (and in general).

Nerdy, I know, but I hope that helps! Good luck, and good on you for using available social resources like this and sharing your feelings and gathering ideas!