To simplify the expression 2a + 3 - 5a + 8b, we can combine like terms. Like terms are those that have the same variable raised to the same power. In this case, the like terms are the ones with 'a' and the ones with 'b'.
Group the 'a' terms together:
2a - 5a = -3a
The simplified expression becomes: -3a + 3 + 8b
So, the correct option is (-3a + 3 + 8b).