Welcome to the Q&A Forum

When I first started doing SEO, I did small niche sites, and to some extent I used exact match domains.  Furthermore, I dabbled in the old black hat arts as well. I can tell you that merging multiple sites into one was the best thing I have done (well, that and going completely white hat).  You can save so much time only focusing on one site, that you can spend alot more time creating better content, and like you say more time increasing the authority of one site rather than having to split your attentions over three. Sure, you may find in the short term that you don't do so well, but if you are wanting to think ahead and consider how you can expand even more with related topics and better ranking through a stronger domain authority then I think migrating into one site could well be worth considering.

Hi Nevil Google does support cross domain canonical tags as they announced here: here http://googlewebmastercentral.blogspot.co.uk/2009/12/handling-legitimate-cross-domain.htm However I am not sure this is the best strategy for you. When you use a canonical tag you are basically telling Google not to index that page but the one in the canonical tag instead. If Google listens to this it means your job pages will not be indexed in Google. The better strategy for this would be similar to Indeed's where you take a snippet of the job, add your own unique content to the page and then have the call to action button link to the original job itself. This should give that page the best chance to rank. Indeed is a tough example to use. They are pretty much number 1 for every job related term with a site that is extremely thin. I believe Google has listed this site similar to Wikipedia where it is a credible source for jobs and they give it additional weight. How fair Google is being here has been a long debate of mine I hope this helps