Final answer:
The three main influences on a person's course of grieving are personality, culture, and biological factors, each affecting how individuals process and cope with loss.
Step-by-step explanation:
The three main influences on a person's course of grieving according to Colin Murray Parkes are personality, culture, and biological factors. Personality traits can affect the intensity and duration of grief, with some individuals being more resilient due to optimistic or positive personalities, while others may experience a prolonged period of grief due to traits such as neuroticism. Culture influences the grieving process through the socially accepted ways of expressing grief and the support systems available within a community, with some cultures providing extensive communal support and elaborate grieving rituals. Lastly, biological factors play a role in how individuals process and cope with loss, which might be evident in the physical responses to stress or hereditary predispositions to mental health challenges.