>My original site I have limited to a mirroring depth of 
>three. How do I mirror the newly added sites such that the 
>mirroring depth is unlimited but does not access links or 
>sites outside the newly added site itself?
You can not mix multiple options / depth level for 
different URLs among the same project. But use of depth is 
discourages, as scan rules are generally better.

> And having fully captured the original site, how do I 
> prevent HTTrack from needlessly checking the original 
site 
> for updates, etc., but turn the engine lose on the new 
> sites added?
"Continue an interrupted mirror".