Help setting up a specific domain mapping set-up

Hi,

I have a multisite configured as a subdirectory install with domain-mapping and multi-domains both installed and activated. No issues with this.

I have a number of sub-sites mapped:

multisite.com
mulitsite.com/sub-site-one <--- domainone.com
mulitsite.com/sub-site-two <--- domaintwo.com
mulitsite.com/sub-site-three

That's all fine and dandy.

How do I set-up the following?

domainthree.com ----> hosted under one directory
domainthree.com/mapthisplease ----> mulitsite.com/sub-site-three

So the above would become:
multisite.com
mulitsite.com/sub-site-one <--- domainone.com
mulitsite.com/sub-site-two <--- domaintwo.com
mulitsite.com/sub-site-three <--- domainthree.com/mapthisplease

Help appreciated on this one please.