The Adafruit Motor Shield has 4 H-bridges. Each stepper requires 2 and each DC motor requires 1. So if you have 2 steppers, there are no free bridges to control an extra motor.
Your options depend a little on what kind of control you need for the DC motor. If you need only speed control (one direction rotation), you can control the motor with a simple MOSFET and any free digital pin. If you need both direction and speed control for the motor, then you need a full H-bridge controller.
Pololu has a range of motor control modules you could piece together to handle a wide variety of motor combinations.
2 stepper motors with ratings of 5V and 0.57A 1 DC motor with rating of 5.6V and 0.67A
which require both speed and directional control
i'm not exactly sure how to implement the MOSFET board to the circuit i was intending to have. sorry but could you recommend a motor shield / motor controller that could be added on to the arduino uno rev3 whichcould do both speed and directional control?
or say give a link to a picture/schematic on how the circuit might look like?
If you need both speed and direction on the DC motor, then the MOSFET solution won't work. You can still control the 2 steppers with the Motor Shield. You could add something like one of the Pololu "simple" motor controllers, or one of the smaller motor driver carriers to handle the DC motor.
Ok so that means if I replace the original one with SN754410 I would then be able to control my stepper? Soooo that means it'd possible?
Haha thanks a lot then. Ok so basically as long as the steppef motors adhere to the ratings required which is 0.6A/phase or in this case a 1A/phase current from the other chip and meet the minimum voltage requirements then it should be able to be powered by the motor shield right?