The question is why are the effects so different between the two injuries?

Help me please

The difference between the effects of a spinal injury at C3 and one at T3 is due to the fact that the segment of the spinal cord affected, and the functions it controls, are different.

Explanation:

From the spinal cord originate the spinal nerves, which are responsible for controlling both sensory and motor nerve functions.  

Each segment of the spinal cord controls a group of specific motor and sensory nerves, known as myotomes and dermatomes.  

Vertebral injuries can cause spinal cord inflammation or permanent spinal cord damage. The involvement of a segment of the spinal cord can be evidenced by  

The difference between a C3 and T3 lesion is that the cervical lesion affects the dermatome and myotome that control this segment, in addition to all those below it. On the other hand, a T3 lesion can affect this dermatome and myotome, in addition to the lower ones, but the functions above the lesion will not be affected.

- A vertebral injury in C3 —third thoracic vertebra— is very high —sensory and motor level with response maintained on the neck— and produces a failure of almost all the peripheral nerves, with paralysis from the neck down. This includes breathing and control of the diagphragm (breathing) and the four limbs.

- Injuries in T3 —third thoracic vertebra— the motor and sensory level is lower, allowing movement of arms and breathing, but with paralysis of part of the trunk, lower extremities and control of the bowel and urinary bladder.

looks like you are missing options or a image.