What is the earliest level a warlock can take Hellfire Warlock prestige class?

Hellfire Warlock Requirement is:

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If I meet these requirements at level 6, can I go into HF warlock prestige class in level 7? I am currently a level 6 warlock however I have 9 skill points invested in Knowledge (the planes) and + 3 is coming from int modifier. Would that make Knowledge(the planes), level 12, or am I capped to level 9 (level + 3 SP) according to PHB?

PHB +3 SP rule:

If you want to pick up a new skill for your character, you can spend skill points equal to his or her character level +3. These skill points buy 1 rank each if the new skill is a class skill or 1/2 rank each if it’s a cross-class skill