I've got 3 different domains that describe 3 types of services that i offer.      The main domain is ranking very well based on the keyword that corresponds and i'm just now building out the other 2 domains.   My question is, some of the content is duplicated just because it's the same overall type of business.   On the duplicate content should i just link back to my main site (is that going to look bad in Google eyes)?  or should i just build out all new content.   

How does reciprocal links within the same IP class get viewed?

Good question....

Gypsy would know better on this subject, as he has several sites that are pretty much rolled into the same general field (seo, hosting, on-line development, etc)

However, I'm sure we all could agree that the dupe content will be frowned upon. Re-writing the content will help it stand out on it's own, and help you hone your writing skills. It's a win/win situation.

Linking back to each site? Think of it this way - If the link is relevant, then you shouldn't have any problems. Try not to SEO the link tho (personal preference). You won't get much value anyways, but keep it clean, honest, and natural.

If you link all the sites together, with natural looking links, then all your doing is promoting your other services... not a big deal.

Exactly how closely related are these sites?

Ya know, there is something to be said for rolling all your efforts into one single site. The long term promotion can be a pain, you could be paid back in conversions. Furthermore, a single site will receive more love (from you), rather than splitting it all up. Double check to make sure you're making the site(s) for visitors, and not just geared around keyword targets.