They are all relatively close to each other - av 4 to 5 km. But if you spend a min of 3 hours (that's my time) that would be impossible. Even 2 hours. Forget travel time.
Thank you so much
The Grand Palace (with the Temple of the Emerald Buddha) is roughly next door to Wat Pho, while Wat Arun is just across the river, more or less. A combination of those three is doable, if you start early. Wat Saket is a little further away. If you are travelling to Thailand quite soon, how you stand up to the heat will decide how much you can do in one day!
After, river crossing ferry back to Wat Pho pier.
Wat Saket in the heat may be too far. But, do not take taxi from near GP/Wat Pho area, or you will be asking to be ripped off. Walk a bit away, then taxi.Edited: 25 March 2018, 22:37
Yes I think you could do them all in one day unless either the heat gets too much or you become ' templed out'. Wat Saket is only 20 baht to get in and has some nice water features with statues on the way up the steps. From the top you get a good view of Bangkok. You can also get a good view of the river from the platform about half way up the front of Wat Arun, if the stairs are open to the public again. I am not sure because last few visits it was covered in scaffolding but I heard the scaffolding has gone now.Edited: 25 March 2018, 22:42