If there are three or more and your new, try doing it the hard way (make sure your patient). Get 15-25 tranq arrows or toxicant arrow ( the same as the tranq arrows but a lot better torpidity), get 100 narcotics just be sure that it doesn't wake up, get 80 raw meat or 10 raw prime meat, and get your argentevis with you.
If find three, try waiting until one splits apart by leading all three of them in a pack of herbivores and they will start attacking others rather than joining with the pack, as soon as the other one is too far from its pack go ahead and kill it and do this one by one to each allows (except the alpha, which is your goal to tame).
Try to make them fall into a ravine. Flyers help with this tactic. Or better, look for some that spawned or fell into one. There’s a ravine on my SP game where there are around 11 allos that fell into it. After getting them into a ravine, kill off all but the one you’d like to tame. Proceed in taming as usual.
It will usually be very difficult to separate one from the pack. I suggest trapping all of them (usually 3 in a pack) in a behemoth gate or ramp style trap and kill the ones you dont want before taming the one you do want.