Trios are possible, but you do have some challenges, especially since they are free-roam, so a third rabbit will be seen as an intruder, in addition to a stranger. This is the same for someone with a single bun, but with having a bonded pair, I do wonder if your rabbits may not be as interested as a single bun to connect with the third rabbit. Is there any chance you can (safely) meet the rabbit with your pair to see their reaction prior to committing? The likelihood of an instant love is… close to zero… but if there seems to be a particularly aggressive reaction from your pair to the third rabbit (or vice versa), it lets you know that it might be tough.
I do not have experience with trios, but a past member documented their journey in trio bonding here: https://binkybunny.com/forums/topic/bonded-trio-journal-luna-atlas-and-nimbus/
… Notably, it seems that, rather than doing dates with the bonded pair and the third rabbit, there are many instances of actually having the third rabbit on a solo date with one of the bonded pair.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.