Custom fields / get_post_meta / get_the_id

Hello,

I think it must be pretty basic question but I am only starting. Can someone have a look at the 3 versions of the same (?) code below and say what the difference is? All of them seem to work fine in the loop I am working on.

Which should be used: $post->ID, $the_ID or get_the_id()?
Is it necessary to have global $post;?

global $post;
$content = get_post_meta( $post->ID, ‘my_custom_field', true );
echo  $content;

or 

$content = get_post_meta( $the_ID, ‘my_custom_field', true );
echo  $content;

or 

$content = get_post_meta( get_the_id(), ‘my_custom_field’, true );
echo  $content;

Many thanks for your help