They're quite steep and will each take you roughly about 45 minutes to get up. On a hot day the third one's a bit of a struggle. I rode my mountain bike, but in Thailand it's probably not the most efficient choice. As a rough rule of thumb, the monsoon comes between about May and October, and the dry season happens between November and March. I like to ride with a bit of cloud cover and think the odd shower or two cools things off a bit, so my preferred riding time is just post monsoon, around October - November.
Most people prefer the dry season.
Temperatures can be anywhere during the day from 20 to 38 degrees Celsius and at night can drop down as far as 15, depending on where you are. One crossing into Thailand at the Malaysian border and the other at SongKhla, crossing the inlet to connect with the main coastal highway. Both run back and forth every half hour. Always a nice experience to stop and compare notes on the road. It's that crazed look in the eyes that gives them away.