There's a lot going on here that you can't see, hunt groups are very sophisticated. Probably the calls ring to a hunt group. If all ring at the same time, the hunt group member is an extension list. Hunt group may have some other magic in it, such as escalation if unanswered (recall), announcement and overflow.
But to get to your basic question, look in SYSTEM/DEVICES AND FEATURE CODES/HUNT GROUPS. Hopefully one will be named something obvious like RING IN or MAIN NUMBER (if not, let me know and I'll tell you how to find it). Within that group go to MEMBERS; I suspect you'll see a number that starts with a P. This is the extension list, and all phones on the list will ring at once. My recommendation is that you right-click in the MEMBERS and add the one phone you want to have ring (in pick list choose 52XX/53XX, then desired extension). You now shoupld see both the PXXX number and the desired extension in the members list. My recommendation is that you just drag & drop the single extension so it's ABOVE the PXXX entry...result will be that calls will ring the single extension and if unanswered or do-not-disturb will the ring all three stations. Ring time is set in the hunt group timer section, called NO ANSWER ADVANCE. Other option is that you could just remove the PXXX entry, but then if the extension is busy or unanswered calls will just stay there.
