「峠{とうげ}を二{ふた}つ三{みっ}つとこえても、まだ海{うみ}は見{み}えてこない。」
こえる=越える
First and foremost, quantifiers are often used adverbially in Japanese whereas they are more often used adjectivally in English. This has been discussed in this Q&A.
Japanese way: 「峠を二つ三つ(と)こえる」、「ハンバーガーを二つ食{た}べる」、「テーブルにリンゴが2個{こ}ある。」, etc.
"English" way: 「二つ三つの峠をこえる」、「二つのハンバーガーを食べる」、「テーブルに2個のリンゴがある。」, etc.
In the sentence in question, 「二つ三つと」 functions adverbially to modify the verb 「こえて(も)」.
There is virtually no difference in meaning between 「二つ三つと」 and 「二つ三つ」. Both function adverbially and both mean "a few (times)".
The 「と」 simply adds a very small amount of emphasis to the quantifier/adverb. Likewise, the difference in meaning between 「ゆっくり」 and 「ゆっくりと」, 「しっかり」 and 「しっかりと」, etc. is minimal.