Create a 3 day HR slope column based on date column in R

Question

Data frame looks like this:

DATE        Athlete    HR
2020/07/31  Athlete 1   56  
2020/07/30  Athlete 1   58  
2020/07/29  Athlete 1   64  
2020/07/28  Athlete 1   36  
2020/07/27  Athlete 1   57  
2020/07/26  Athlete 1   66
2020/07/25  Athlete 1   69  

Have looked into rollapplyr() and slope() but unsure on how to use these functions in order to get a 3 day slope for HR data column using the dates column?

Was thinking that maybe I would have to create a function and use slope() within the function in order to calculate the 3 day but unsure how to do it.

Thanks in advance.