I currently have three websites all in one niche: a blog, forum and tutorial site (each linked through a navigational menu as if to appear as one site). All of these are under different URL's therefore acquiring PR for each domain would be a lengthy process, would it be better instead to buy another domain and turn it into a landing page for all three of the sites so I can concentrate on building one pages PR rather than three?
I believe you need to retain that structure as it will help you to build PR from one site to another. creating one landing page will actually make it more complicated by adding one link infront of three legitimate links.
More site means more work and effort and I think it is not recommended since you sites are related. I would choose one of them as main page and it a good portal towards other pages/sections.
I also wouldn't recommend creating one landing page for all 3 sites. You may want to include a link to Sites B and C for example in Site A, etc. A landing page on it's own won't increase traffic. SEO and marketing increases traffic.
linking all sites to each other will help you to build PR and it will be easier for you to forward or redirect your traffic from one site to another, in addition to that you will be gain benefits by sharing PR among your sites.