The UK and the US begin and end summer time (Daylight savings time) on different dates.

For a few days every year, New York is six hours behind London instead of five.

So in October and March the schedules get a bit messed up. The flight length will be the same, more or less.

As for arrival into JFK, the earlier arrivals tend to have shorter queues, so you are at an advantage. That said, there are no guarantees.

BA/AA would accept your connection with two hours between flights. Having three hours means you have a cushion. It's likely you will be fine.