No, DNA polymerases cannot fill in 3' overhangs. To create blunt ends 3' overhangs must be removed. DNA Polymerase I, Large (Klenow) Fragment (NEB# M0210) or T4 DNA Polymerase (NEB# M0203) are the best choices to remove 3' overhangs. Mung Bean Nuclease (NEB# M0250) can also be used to remove 3´ overhangs.