please help, doggy won't poop

Sorry I can't type Chinese here.  My schnoodle girl hasn't pooped 3 days. I found my terri-poo boy wiping his bottom on the lawn, seemingly trying to poop but only a little bit came out.   They ate a huge steamed bun, which is not their usual diet : (   The vet  won't be able to see them until tomorrow evening.   Anything I can do to help them now?  Also, can I do anything to prevent this in the future?   Any suggestion would help. Thanks~