I'd avoid like the plague, sorry! If the rescue doesn't know if they've been spayed, then that means the rescue hasn't spayed them...and the chances of a pair of bunnies coming into rescue already spayed (and the rescue not knowing this) are virtually zero. I'm afraid that combination is a recipe for disaster, and is most likely to end up with injuries and more than likely three single bunnies at the end of the process, as two unspayed females are very likely to turn extremely territorial when there is a newcomer and will probably not just fight with her, but also with each other.
If you've got your heart set on them, then I would suggest the only sensible option would be to get them both spayed (or, being optimistic, getting the vet to confirm for certain whether they are spayed already), wait at least 6 weeks for hormones to die down and then attempt the bond...there is always the risk of course that the bond doesn't work and worst case scenario is that you could end up with 3 single bunnies. In all honesty, I'm not sure it would be in the best interests of the existing pair to try it.
Personally, if you're going to go for a trio then 3 girls would be my least favoured combination. It all depends on the bunnies of course and three girls can work, but in my experience, girls can be the trickiest to bond successfully even if all are neutered. In any case, it is absolutely essential to ensure that all bunnies are definitely neutered before trying. Adding a single bun into a pair is very different to having a trio which comprises siblings/family members together as they will always have been together and will have a familiar smell and look.
Good luck with whatever you decide.