What would they do?

[Belegorn]

The One corrupts. Look what it did to Saruman and he did not even lay eyes on it. By the time of the War of the Ring it appears that Galadriel at least had accepted that it had to be destroyed even if doing so resulted in the possible breaking of the power of the Three. I do not think they were quite sure if the Three would lose their powers had the One been destroyed. We know that two of the three keepers of the Three would have it destroyed at least at that moment when Sauron was cast down.

Originally Posted by Silmarillion; Of the Rings and Power and the Third Age

Isildur would not surrender it to Elrond and Círdan who stood by. They couselled him to cast it into the fire of Orodruin nigh at hand, in which it had been forged, so that it should perish, and the power of Sauron be forever diminished

At the Council of Elrond, Elrond told them that Isildur would not heed their advice. So I'd have to say maybe two of the three would have had it destroyed. I'm not sure about Galadriel. It seems she came to her conclusion when she met Frodo. Elrond said is was a danger to all. So I'd think he would not have it protected or hidden, but rather destroyed as we can see he and Círdan both sought for.