If you did not install the patch, the teleporter stones were removed from your inventory when entering act 3 due to a bug.

BTW, the teleporter stones are removed by design when you enter act 4, and you do not get them back. <img src="/ubbthreads/images/graemlins/cry.gif" alt="" /> The imp historian said he would be back for them, though there is no cutscene or explanation in-game to explain that that is what happened.