First off, $25 for those skis is a awesome deal nice job. Just be careful, $25 for skis with a perfect base almost sounds too good.
People will tell you varying answers for this, a 3rd mount is pushing it but not unheard of. You can mount bindings in the same holes as previous if the bindings have the same screw pattern as the previous mount (helicoil).
A shop tech will most likely tell you not to, but it's ultimately up to you.