Newer roundups allow using the javascript based classhelper to select a single item.
The fix described in issue2551420, makes it suitable for link (not multilink) properties.

You would use a field (text) input rather than a select and pair it with
roundup-classhelper with inputtype="radio" that allows selecting only one item.
You can add a search by company name to the classhelper which should be a lot faster
than transferring 3000 options in the page.

See the checkin comment for changeset:   8722:d7ba8e635d4e.

Also I added support for names with commas in changeset:   8723:02821a3d6f7a.