Two or more adjectives that do not modify the noun separately are cumulative.
Three large steel panels protected the undercarriage.
Beginning with the adjective closest to the noun shapes, these modifiers lean on one another, piggyback style, with each modifying a larger word group: Steel modifies panels, large modifies steel panels, and three modifies large steel panels.
Cumulative adjectives cannot be joined with and (not three and large and steel panels). Though commas are required between coordinate adjectives (those that can be joined with and), they do not belong between cumulative adjectives.
Exercises:
Misuses of the comma 1
Misuses of the comma 2
Related topic:
Comma between coordinate adjectives