More sons of Philip II of Spain survive

It seems to me that the Habsburgs of Spain after Philip II were on the perpetual decline, Philip III was quite incompetent, and Philip IV seems to have been riddled with numerous issues. However, Philip III had three older brothers who all died before him: Ferdinand born 1571 died 1578, Charles Laurence born 1573 died 1575, Diego born 1575 died 1582. And then there was of course Carlos the mad, Philip II's eldest son by his first wife who died long before this.

Diego was said to be quite smart, knowing how to speak multiple languages at such a young age and Ferdinand was said to be reasonable smart also, what might have happened for Spain had Philip's older three brothers survived. So, this being Ferdinand, Charles Laurence and Diego, what sort of careers could they have had, Ferdinand being the heir, would naturally be groomed for the throne, Charles Laurence perhaps the military? Diego, the same? And Philip perhaps the church?
 
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