The rectifier switches (SCRs or IGBTs) are likely the only components that will be similar between AC drives and DC drives. There will be many differences some of which are the design of the energy storage and filtering components (inductors, capacitors), the control of the DC section and the inverter control and switching in the AC drive, the design range of the DC bus voltage (for the DC drive it needs to go to zero; for the AC drive it does not).
An AC drive does contain an intermediate DC link that is inverted back to the speed or torque control AC power supply for the AC motor. The drive electronics varies and monitors the frequency, voltage and other characteristics of this output to control the speed or torque of the motor. While the DC link voltage is variable, there is no direct way to control the voltage to achieve speed/ torque control of a DC motor.
Depending on the type of DC motor you have, it could be possible to power it from the intermediate DC circuit of your "AC Drive", but not all such drives do actually have a "proper" intermediate DC stage, and even for some of those that have, this stage could be floating relative to your earth potential, so it could be very dangerous, if your DC motor is mechanically connected to virtually anything. For such a small drive, like 3 hp, many would prefer the practical approach of ordering a new spare of each type.