There's two possible options at work here. If M3 is based off the A16 cores, then it would be a 5nm part. However, if it is 3nm as widely predicted, then it would likely be based off A17 cores instead. Given that both M1 and M2 came out after their iPhone equivalent hit the market, we could expect to see the A17 hit the next iPhone, then M3 come out within 6-8 months of that (depending on yield rates and how many TSMC can reliably produce).