If you really need three discrete instances, you might want to look at using vmware to set up three separate virtual machines, each with their own IPs etc... on that one box.
This was my thought; depending on how much RAM you have in each blade, you could have two virtual servers on each blade, one server being HTTP and one being Postgres.
I'm not sure how big your application is, but if it's not the next SAP then you might be able to get away with as little as 1G of RAM per blade (on the dev and QA instances anyway), if you virtualize Linux servers using XEN. That's the way I'd do it.
Cheers, -J
