Append ‘Continue Reading’ link if post length is more than 3 lines of text?

Instead of doing the lines, I would try to go for characters or words. The example below uses characters.

Don’t take my word on it as it hasn’t been tested.

$character == 40; // number of characters to compare
if(mb_strlen( get_the_content() < $character){
the_content();
}else{
the_content('Continue Reading...')
}

Alternatively by word count.

$words == 20; // number of characters to compare
if(str_word_count(get_the_content(),0)  < $words){
the_content();
}else{
the_content('Continue Reading...')
}