Hi @codeworks,
I assume you want to add the 3 to the existing value which is already stored in the DaysEarned field?
In that case you can use an add function in an expression to calculate the new value and use it in an update item action. You can first retrieve all existing list items and loop through them with an apply to each.
Below is an example
The DaysEarned field uses the expression below
add(items('Apply_to_each')?['DaysEarned'], 3)
