If I'm in the process of becoming executor of my father'ss estate and he had 3 vehicles valued at less than $500 each, can my stepmother sell these vehicles now?

If you are named executor in your father's will and you agree that the vehicles should be sold, then your stepmother can go ahead and sell them.  As long as the actions benefit your father's estate (and it sounds like they do), you will have authority to "ratify" the sale after you are appointed.  However, someone will have to obtain authority to transfer the titles to the vehicles and that may be impossible until you are actually appointed executor.