Forming each element of p by direct multiplication of all but one of the elements of a is wasteful ($O(n^2)$ in operations), so first we'll calculate the product of all the elements in a and form p by dividing this product by the appropriate element.

a = [1, 2, 3]

prod = 1
for e in a:
    prod *= e

p = []
for e in a:
    p.append(prod // e)

print(p)