Can you be a rebound in a poly relationship?
I don’t see why not. Polyamorous relationships have not magically transcended all of the other junk that sometimes comes along with relationships. People in polyamorous relationships are capable of being abusive, telling lies, leaving passive aggressive post-its, taking advantage of people, texting while driving, or otherwise making bad choices.
If you’re worried that someone you’re dating is treating you like a rebound; if you’re worried they are not committed to you or willing to meet your emotional needs; if you’re worried they are using your time, emotional labor, sex, etc. to soothe the pain of a breakup in ways that you’re not okay with - talk to them about it! Let them know your concerns and needs and expectations and give them a chance to process this with you.