Finn was named in an unchanged side on Wednesday but took only four wickets in the first two matches against a touring side struggling in the English conditions.
"Stephen when he is absolutely on song is I believe up there with our three best seamers in the country," Cook, who became the first English player to reach 10,000 test runs in the second test in Durham, told a news conference on Wednesday.
"We saw last summer against Australia when he bowled fantastically well, and we saw this winter he was outstanding. It's not always perfect. But we know Stephen has the ability to take wickets and when he clicks he's a hard bowler to face."
"He hasn't quite nailed it these two tests but I've seen glimpses of it in the nets. I've got a big bruise on my leg to prove that," added Cook.
The hosts are looking to complete a first whitewash since a 4-0 rout of India in 2011.
(Reporting by Martyn Herman; Editing by Robin Pomeroy)
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