The reason why you saw no traffic in
Wordtracker for "c d e" is b/c the exact phrase may not be used in WT's sample data** but Google attributes certain variations of "c d e" to your exact phrase "c d e".
You can use
Trellian KeywordDiscovery and select phrase match to get an idea of how much volume is available for "c d e" and it's phrase variations. You can even use the other keywords that pop up for your "c d e" phrase to increase your theme quality score.
**Wordtracker relies on sample data from meta search engines and directories like dogpile. This gives you a more realistic idea of how much volume your site will get.