I am planning to change 3 website domains. aaa.com; bbb.com; ccc.com. Which now have good search results with keywords aaa; bbb; ccc. If i change aaa.com to ggg.com and put all titles in and change backlinks to ggg.com this will be usual. And google will treat it as normal change. but if i change also bbb and ccc to ggg.com. ggg.com would looks like "aaa bbb ccc website". Is this possible? How google will treat such change? (ggg will get new backlinks with aaa anchor, bbb anchor, ccc anchor) --- another question. Is it necessary to get old domain for change? Should ggg.com be an old domain. Or there is no difference if new domain is old or new, in any case ggg.com will loose all his old SEO results.
You need to 301 the old domains to the new one. This way all link juice and traffic will be redirected to your new Domain. If you can find an old Domain that is what your looking for (keyword/niche) then yes it's better than a newly registered Domain. If you have good backlinks to ggg.com then keep it and use it. If it has backlinks then it would be better than an old domain with zero backlinks.
The full link juice will not pass to the new domains even with a 301 redirect, but the loss of link juice value is usually pretty minimal, but something to keep in mind. http://www.seroundtable.com/archives/021832.html