Ismay was technically "just a passenger" and I bet he was allowed on the bridge whenever he felt the urge to "visit."It would have been impossible for John Jacob Astor to be on the bridge, as passengers weren't allowed anywhere near that area. It was a rule that was put in place and still exists today (unless you're invited by the Captain), so Titanic's officers wouldn't have allowed it.
As far as Thomas Andrews goes, from what's been discussed in previous threads on here, he wasn't on the bridge. Instead, Captain Smith more than likely went to visit Andrews' cabin.