It's a bit up and down, and the signage is less than perfect, but assuming you don't get lost it will take 10 to 15 minutes to walk from T3 to the rail platform. If the weather is nice I sometimes just through walk outside, albeit at risk to life and limb. So assuming your flight is more or less on time and there's no hold up on baggage delivery, a one hour lag should be fine. There will always be exceptions though, so be careful about non flexible rail tickets.