You need to do a /castsequence I.E

#showtooltip
/castsequence reset=combat/target devastate, shield slam, revenge

So one key will do all 3 in that order over and over

I would not advise having a "proc" like revenge on the same key as devastate and shield slam though as you'll get stuck doing nothing until revenge procs : /
Also shield slam has a 3 second cool down I think so you need more abilities between the first slam and the next.