I understand that if a college asks for three recommendations they have to be from three different teachers. If there are two colleges that ask for three each, can a teacher submit the same recommendation to each college?
Yes. If you apply on the Common App, the same teacher recs will be sent to the multiple colleges you apply to.
Yes. This is by far the most sensible and easiest way to do it. (Fewer teachers to ask!) . These letters can be helpful for scholarship applications as well.
However, if there is some reason you’d like different teachers for a specific school (school requires certain type of teacher for example), you can do different teachers to different schools.