You'll certainly be able to use ETTL with the 600EX-RT and ST-E3-RT. However, you won't be able to use the ST-E3-RT to control your 430EX II units, as it lacks optical triggering. You could use the 600EX-RT to control those 430EX II's optically, including using ETTL, but that kinda defeats the purpose of buying the radio-capable units.

Some options:

• Skip radio for now and stick with your 430EX II's. Pick up a used or refurbished flash that you can use as an optical master -- the 580EX, 580EX II, or ST-E2 would all work. Canon's tiny 90EX can be used with some bodies as a master; I don't know if the Rebel XS is among these, but if it is then this is a very inexpensive way to go.

• Go with Canon's radio system and start to phase out your older flashes, or keep an eye out for ways to make them radio-capable. I read recently that Yongnuo will introduce a receiver that will work with Canon's radio system, so you could use your 430EX II's with the 600EX-RT (without switching to optical triggering). (Sorry, can't find the link right now.) The down side is that each receiver will be priced around $90.

• Upgrade your triggers. The reason you're not able to use ETTL with your current speedlites is that the Cactus V5 triggers don't do ETTL. But there are existing radio triggers that work with ETTL, and switching to these will solve that problem for you. However, those triggers aren't compatible with Canon's radio triggering system.