I found out while trying to make a COM+ installation that WiX3 doesn't yet support this. I was working on it originally with the latest WiX2 drop and ran into some difficulties (my own doing it turns out). While troubleshooting, I thought I'd give the latest 3.0 drop a try just in case some bugs had been fixed.
After backing up my *.wxs files and running them through the new wixcop using the -f switch to upgrade the schemas I received compile errors around the pubca extensions. Looking around, I found there wasn't any binaries for this in the drop. My next approach was to browse the CVS repositories thinking maybe they just weren't included in the packaging yet. Nope.
I jumped on the wix-devs mailing list and asked about the missing pubca stuff in Wix3. Kudos to Rob Mensching who responded within 30 mins. with the answer...the pubca custom action stuff has yet to be merged into the WiX3 codebase. So...if you're doing COM+ stuff you'll have to stick with WiX2 for now.