grammatical number - Singular or plural verb after a list containing three items?

I'm trying to clear this up for a client. He's using the slogan

"When Time, Quality and Value Matters – Count on TEAM."

"Matters" sounds completely wrong to me so I changed it to "matter" on his graphics temporarily, but he's insistent that it's "matters". I know that when you break the sentence up into parts, the verb used would be "matters", but does that change when the 3 items are together in one list?