The GM decides
Seriously, that has no clear answer in the system, and every GM you ask will have a different idea of how to handle that. Some will say you can gain as many levels as you have earned XP for them, others that you can only level up once per encounter, others that you will be 1 XP away from the next level.
All we got is this, under Character Advancement:
A character advances in level as soon as he earns enough experience points to do so—typically, this occurs at the end of a game session, when your GM hands out that session’s experience point awards.
And this, under Awarding Experience:
Characters advance in level by defeating monsters, overcoming challenges, and completing adventures—in so doing, they earn experience points (XP for short). Although you can award experience points as soon as a challenge is overcome, this can quickly disrupt the flow of gameplay. It’s easier to simply award experience points at the end of a game session—that way if a character earns enough XP to gain a level, he won’t disrupt the game while he levels up his character. He can instead take the time between game sessions to do that.
So, we know the GM decides how, when and how much XP to reward characters. If he says you can level up 3 levels at once, by any means go for it, there are no rules against that.
Personally, since you seem to be joining a 7th level party, I would completely allow it, but if you are a beginner GM, I would advise against uplifting characters like that because they will likely lack any real experience with their characters (too much power in inexperienced hands) and likely face a terrible death at higher level enemies before they had a chance to actually learn how to use all their abilities and tools.
