The course is supposed to be fully decorated (flowers, filler, groundlines) prior to ground jury & TD inspection, which happens before the 3pm deadline. The TD & ground jury can request changes to the decorations as desired-- more obvious groundline here, fill in that gap at the base of a table, request to remove overly bright flowers from the BN coop, etc. Some TDs prefer sparse decorations to avoid distractions; others want LOTS of flowers to make the groundline more obvious, or taller plants to fill in empty space below a hanging log, for example. All these things should be completed prior to the course open for competitors.
HOWEVER, as competitors inspect the course, they should contact their rider rep with any concerns. I’ve noticed holes, or flagging issues (particularly with corners), and brought this to the rider rep, who then meets with the TD to discuss the issues. So yes, at times, the course may be altered after 3pm, if riders notice something wrong that needs improvement.