Do you sending traffic from the Big 3 search engines to the same landing page in a niche, or a separate one? The same domain or a different domain for each search engine?
There's nothing stopping you from doing what you want. If you want to use different search engines to drive traffic to the same page, do it. If you want to use different pages for each search engine, do it.
If you are using different pages (but presumable the same domain), are these landing pages islands or are they linked by navigation to the main pages of the site? If so, are they duplicate pages? I am thinking about how you might fit them into a site, without triggering dup content filters for Adwords.
Hmm, for basic arbi landing page targeting for organic search is pointless unless your CTR is really low and you have lot's of visitors. Then you could always do splittesting if visitors from certain SE gives better CTR with another landing page. But I wouldn't bother if site is just giving basic CTR (around 50%) and clicks are low (something like 0.50).
Basically I use similar landing pages for every source. But I I vary each page for specific ad groups. For example - "Gardening Hoses" and "Gardening Supplies" would have separate landing pages.
The main finding I got is that Google visitors from PPC convert almost as good as from Adwords. Visitors from Yahoo don;'t convert anyway as good as Google. And MSN plain sucks so we stopped advertising with them.