The look like pulleys, but sailors call them blocks. The one in the middle looks okay in this picture. But it is actually broken, also.

They're part of the mizzen mast. One helps put some purchase on the mizzen sheet. The other two help put some purchase on the running backstays.

Fortunately, we had a spare. We found it -- quite by accident -- cleaning out some drawers. The other two, however, require YATTWM (Yet Another Trip to West Marine).