Welcome to the boards.
You know, for me, this situation isn't really a "is a threesome the first time a good idea" question, so much as that THIS situation, THIS threesome setup, specifically, sounds like a really bad setup to me, for probably everyone involved, in a few different ways:
Personally, and as a general rule here at Scarleteen HQ, I am not a fan of sex presented as a gift or a present.
It's something that can really put the person it's being offered to in a pretty uncomfortable position, especially if they don't want that "gift," or don't want it that way. Imagine having to be the person who has to tell two of his best friends that he doesn't want blow jobs from them on his birthday (or not from both of them), and that maybe he's even sad his friends are coming on to him or see him in a way he doesn't like. Imagine how hard it is to tell your grandma or whatever you just don't want the sweater they gave you. Now imagine how much harder this is. Eep.
This guy would also experience finding out his two best friends have been making sexual plans for him without even asking if he wants those kinds of interactions with either of them. There can even be a real creep factor to finding out your friends have been planning sex about you without you. (Put the shoe on the other foot with say, two of your guy friends doing this about you, and you'll probably get what I'm saying.)
It also can set up a dynamic that makes it harder to everyone involved to really feel free to say not just yes, but also no. For example, let's say you do this, and your friend really wants something in a specific way that actually doesn't feel good to you emotionally or physically -- you're likely to be inclined to try and just suck it up, because it's his birthday, right? It also sometimes happens that people have the idea of giving sex as a birthday gift, then on the day, they aren't feeling it anymore, but can feel self-pressure to just go through with it because that's the "gift" they had. He is likely to feel a lot of pressure to say yes and do what you two want, both because it's set up assuming he even wants this (why?), and there are a lot of messages guys get that say that if they don't do something like this when offered, their masculinity is in question. So him saying yes when he doesn't want to or isn't 100% is pretty possible, too.
There just seem like there are so many ways this could be a truly awful birthday, not a happy one. For anybody..
Lastly, it kind of sets sex up as a thing, a transaction. Which isn't usually so great. To be honest, even talking about a person the way you and your other friend have been kind of objectifies this guy himself, so this whole thing to me feels like it's leaning on some problematic dynamics that would make it way more likely -- specifically -- to be a bad thing for one or more people involved. You know what I mean?
I'm down to talk with you about if a threesome -- which, when it goes well, is usually something *everyone* involved has a part in asking about, planning and negotiating from the front, rather than someone being left out while the other two people plot without them -- is a way you want to explore some sexual firsts, if you want to talk about that.
But I'd just advise anybody that this particular setup is a pretty crummy one to have a threesome in no matter what someone's previous experience is, or even just a twosome a lot of the time.