[Homepage] Update resource cards to three across

[mjabbink]

We should update the ability to compose our resource cards to three columns across vs. only two. It was always meant to be this way and somehow never built to do so.

current

expected

[mjabbink]

FYI
@oliviaflory
@RichKummer
@kennylam
@alisonjoseph

Related to this issue in Carbon for IBM Products
https://github.ibm.com/CDAI-design/pal/issues/1516

[alisonjoseph]

This is easily updated now with a class name change on the wrapper Row around the ResourceCards (if you're on the latest version of the Gatsby theme)

https://gatsby-theme-carbon.vercel.app/components/ResourceCard#group-three-across

<Row className="resource-card-group-3-across">

[mjabbink]

thanks @alisonjoseph!