I need to get one of the images to background-size:100%; but keep the other two as auto. I'm unsure how you do this shorthand.
background: url(/assets/images/site/bg-hero-l.svg) left top no-repeat,
url(/assets/images/site/bg-hero-r.svg) right top no-repeat,
url(/assets/images/site/hero-banner.jpg) top no-repeat fixed;
The
<bg-size>value may only be included immediately after<position>, separated with the/character, like this:center/80%.
So you need to do something like this depending on which of those backgrounds need the 100% size:
background: url(/assets/images/site/bg-hero-l.svg) left top no-repeat,
url(/assets/images/site/bg-hero-r.svg) right top no-repeat,
url(/assets/images/site/hero-banner.jpg) top/100% no-repeat fixed;