Use the word "for" instead of "per" in subscription

Is it coming from the setting field at:
Admin page > WooCommerce > Settings > Tax > Tax Options > Price Display Suffix

If not, try this custom css:

.subs:before {content:"for 3 months"; visibility:visible}
.subs {visibility:hidden}

Thank you! The CSS worked.

But it will be a problem if one of the course is for 4 months. So, is there any other way? Replacing the word in the template?

That could be tricky.

Find out the product id for the 4 months’ course. If its say 2143 for example, use this css as well:

.post-2143 .subs:before {content:"for 4 months"; visibility:visible}
.post-2143 .subs {visibility:hidden}