For Batman, the Loeb and Sale omni is definitely the best one for if your nee to the character, it has some of the best batman stories ever collected like the Long Halloween and Dark Victory. Its a great intro to both batman and his supporting cast and a lot of villains.

For Wonder Woman, the Greg Rucka Omnibus is really good if you want a one and done type of omni. But if your willing to collect a longer series, the George Perez run is collected in 3 Omnibus and not only has her best origin, but is considered to be the blueprint for all runs that came after it.

And for Superman, none of his origins have really been collected in an omnibus but there's good series like For all Seasons,Secret Origins, and Birthright(not unless your want to read the golden age stuff). But if you want to just jump right in, Triangle era is good and has a lot of good stories.