I'm no expert but once my LO was 3 and sleeping in a bed I figured he needed the same tog duvet we did. He has a 4 tog in summer (replaced by a sheet only in v hot weather)and a 10.5 in autumn/winter. he also has a cotton sheet year round to pull over his head! because that's what he likes. I live in NE England and we have had several nights of - 10c. We don't have the heating once he's gone to bed (generally) but due to the extreme cold (plus his bed is up against an exterior wall) I've left it on low until midnight - I have been throwing a fleece blanket on top of him before I get in bed and the heating comes back on full blast at 5.30 am. I guess what I'm trying to say it that at 3 yo he will wriggle out of the covers if too hot and snuggle down if too cold - not like a baby who is helpless and liable to overheating.
it depends on what sort of bed he is in. A cot bed/toddler bed should have a maximum tog of 4.5 whereas a full sized single can have up to a 10.5tog duvet on it. (something to do with the surface area covered by the child). Both my kids are in single beds and currently have 10.5tog duvets - they also take a hot water bottle to bed with them at the moment. (are in Scotland so very snowy and icy just now)
My DS's (2 and 4) both have 4 tog duvets and sleep in cot/junior size beds at the moment. They also both sleep in long sleeve / long leg pyjamas. Also we have the heating on constantly in our house so its always fairly warm.