What is the difference between verhelfen, helfen and behelfen? How are they used in a sentence?
The previous answer already offers a few examples. As for your question about the differences:
helfen is the general term for helping in the sense of doing something or supporting someone.
verhelfen means to take care, that someone achives or receives something he or she wants (as in helping somebody to escape/to be promoted “zur Flucht / Beförderung verhelfen”). It can also be used for the third person (like helping to enforce something “zur Durchsetzung verhelfen”). It implies active effort to a means.
For an example, you can help someone having pain to do a simple action such as getting up (aufstehen helfen), or provide relief (zur Linderung verhelfen). Helping him or her to get to the doctor can be both, depening on what you want to stress (“zum Doktor verhelfen” or “helfen, zum Doktor zu kommen”)
behelfen indicates a workaround, usually in the sence of setteling for less. If you do not have or get what you need, you have to “help yourself out with what you got” … that’s what the help means in behelfen. It doesn’t necessarily mean that the outcome is not as good, it can also just be other than the desired solution (or way, i.e. using a trick, presure, less quality or more resources).
It can also be used as a noun. For example a Rechtsbehelf is the text on official letters from the city or tax services that gives you basic advice about your rights, in the expected absence of a lawyer.