I'm using IRF3710 MOSFET H-bridge for 40A 12V motor driver. Should I use a schottky diode inside or should I use a separate one?
The IRF3710 has a body diode inside. See: IRF3710 - Datasheet
Diodes parallel to the MOSFET's (see schematic below) will ensure that the peak transistor voltage is limited to the input DC voltage and will also provide a conduction path for the magnetizing load current. (See: R. W. Erickson and D. Maksimović, Fundamentals of power electronics. New York: Kluwer Academic, 2004.)
The connection of bulk an drain in a MOSFET results in a body diode. Thus, a MOSFET always has such a diode. (Information about the Body Diode on Wikipedia)
Body diodes can be used in applications. However, usually they have a high reverse recovery time. If needed the body diode can be blocked with two diodes.