Hello Ciaran,
today I added a new google font to the googlefonts.php:

$googleFonts['Source+Sans+3'] = 'Source Sans 3';

I have choosen it in the "Typography Styling".
In the source code of the website I see CSS like this:

h1 {

	font-family:Source Sans 3, Arial, Verdana, sans-serif; 

}

This does not work, I get a serif font because the font needs to be written like this:

h1 {

	font-family:'Source Sans 3', Arial, Verdana, sans-serif; 

}

If I choose another font with more than one word, like Source Sans Pro... it works fine...
It is written with quotation marks.

Maybe the number in the font name could be the problem?

I can use a workaround and set the correct values in the custom.css but thats only a workaround, not the solution.

Can you pls check this? Maybe its a problem in most of the newer templates?

Thanks in advance
Oliver