It's 3-chloropropene.
Since there are three carbons in propene, there are only two places where you can have a C=C double bond. However, both C=C bonds will include a terminal carbon bond, so will both be 3-chloroprop-1-ene. (Another way to think about it is, the two possibilities are identical because flipping one molecule sideways will result in the other.) Hence, because both variations are essentially the same positional isomer, we call it propene without the need to specify carbon number.