你的三个句子正误如下：

(1) To walk often can make us happy. (正)

(2) To often walk can make us happy. (误)

(3) Often to walk can make us happy. (误)

频率副词通常不放在在不定式中间（to与动词原形之间，否则构成所谓的分裂不定式），也不放在不定式前面，而是放在不定式的后面。再如：

What he likes best is for her to call often. 他最喜欢的是要她常给他打电话。

Drinking too much often results in a loss of libido. 饮酒过度经常会导致性欲丧失。