Mercule
Adventurer
Straightforward answer: It probably wouldn't break anything, especially if it was just "credit" from higher levels. By that, I mean that you wouldn't actually get any more hit points until you advanced enough that your total would be higher.
Better option is to give all PCs third level hit points at first level, then they start gaining normally at fourth level. Similar results, intuitive, and easy math.
Even better, just start everyone at third level. It's been said that third level is the new first. If you have concerns with survivability, just run with it and ignore the problem levels. Honestly, the PCs wouldn't even be seriously impacted by still just giving them the PHB starting wealth -- if it really bugs you, give them each an extra 150 gp, which is probably extremely generous, based on my experience.
As others have said, I wouldn't jack with the system, in this way. I'd either start at first or third, with a slight preference for first.
Better option is to give all PCs third level hit points at first level, then they start gaining normally at fourth level. Similar results, intuitive, and easy math.
Even better, just start everyone at third level. It's been said that third level is the new first. If you have concerns with survivability, just run with it and ignore the problem levels. Honestly, the PCs wouldn't even be seriously impacted by still just giving them the PHB starting wealth -- if it really bugs you, give them each an extra 150 gp, which is probably extremely generous, based on my experience.
As others have said, I wouldn't jack with the system, in this way. I'd either start at first or third, with a slight preference for first.