I don't think I'd try 3 rows of seats, seems an awful waste of space, what's your current boats' capacity? That may be a good reason to NEED a larger platform.
Converting to a bowrider won't change the person capacity of the boat, just the # of choices available to sit in.
Doesn't matter if it's a 60lb kid or an adult, it's a combined total: X #of people up to a set weight limit. But if you reach the max capacity of people before you reach max weight, it doesn't matter that you still have 500lbs of weight capacity available.
If you can accommodate the whole family and friends in the current boat's capacity, you might consider a large U shaped bench that wraps from where the port fold out back to back seats stops when open around to the same place at the helm.
My $0.02