Things WILL change, for sure. Also, you said you don't want kids, meaning that this won't be a kid the three of you raise together as equal parents, but their kid and you'll be more of and aunt figure, right? (I'm assuming you're female, correct me if I'm wrong. Sometimes I don't remember things like that very well).
First, you need to be clear about the fact that you don't want kids of your own. Then, add that you do want to support them and that you love them (unless you don't want to support them and their decision appalls you. In that case though, make sure to talk to them. It will probably mean the end of the relationship - I doubt they'll be willing not to have a kid for your sake - but it will be better in the long run for the three of you as well as the baby).
Then, you need to realise that everything will probably take second seat to the baby. Your relationship with either of them, their relationship together and the relationship as a triad. So at least don't take it personally, their own relationship is also going to suffer, it won't be all on you.
On the other hand, being 3 adults provides a significant advantage. After the very early stages, there will be one adult who can take care of the baby while the other two are out together. That does mean you should expect it to be you every so often, however it should not be just you either.
And you should also try and date with the three of you again as well (I assume you do that) by having someone else take care of the child while the three of you are out together.
From your post it doesn't seem to me that you're living together. I would say, there is a risk for the relationship depending on where things go. It might seem to them that you're not being supportive enough, letting them deal with the bad things (taking care of the baby) and wanting to be there for the good ones (the relationship).
You might feel like they're excluding you from their lives, or feel like the baby was their decision and it's unfair of them to ask you to contribute so much.
You'd probably feel much more included if living with them, as they're unlikely to have much time to see you. Consider being the one to come visit them and rarely if ever the other way around.
Either way, it's hard to tell before it happens. And it might work very well. They might find in you some relaxation from their parenting issues, you might enjoy your role as an aunt.
But most of all, I would suggest you be honest. First with yourself: are you going along because you feel you don't have a choice, or are you fine with the idea? Are you wishing they wouldn't have a baby, and just staying because you'd rather be with them than without, or are you just worried because it's a big decision, but ultimately fine with it?
Then, be honest with them. "I'm worried things will change too much, it's a big decision". Or even "I'm really not sure I feel up for this", if that's the case. I think talking with them could help, you could talk about what is going to be expected of you towards the baby, for instance.
And don't forget the pregnancy. She's going to need support during her pregnancy, and she might feel tired or not want to be intimate with either of you (or want more intimacy than before. You never know). You might find that being together (without ignoring her when she needs help, of course) will help you deal with that, and that you'll also be support for one another. I've heard that fathers often feel excluded while their female partner is pregnant, at least there will be two of you this way. (Just be careful she's not the one who feels excluded, either).
And for that matter, the baby tries. It will become important that he be with her and not with you during her ovulation period for instance. You'll have to plan the scheduling around for that.
In other words, lots of potential changes, lots of potential messes, but you should be able to prepare for it and keep your relation strong.