My youngest was my latest and leastmotivated to learn to dress himself - ppretty sure there were 3 factors
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When his older siblings were 2-3 we didn't have to rush to get anyone to school, so I could just wait til they dressed themselves and not get stressed. Since DC3 was a few months old we've needed to be quickand eefficient in the mornings meaning I dressed him for too long to keep things moving swiftly
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He has too much help - if I get on with something he'll often ask an older sibling to help, and they both rather like dressing him
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He had too many clothes which were not easily accessible to him to help himself and make his own choices from (partly due to having so much passed down in addition to things bought for him, partly due to space)
Solving 3) and having a word with siblings only to help him if he was actually stuck were the break through for us.
I sorted his clothes into little baskets on shelves - a basket of pants, a basket of socks, a basket of short sleeved T shirts, a basket of long sleeved, a basket of jumpers, etc. Etc.
This makes it much easier for him to select his clothes and dress himself than when some things were on hangers and some in drawers etc. Etc.
My eldest dressed herself at 2 and my DC2 just started did it of his own accord too, maybe a bit later but before turning 3. DC3 has been later with all the self care stuff, but since I sorted his baskets he has always dressed himself completely, from just before he turned 3.5. He has mainly "easy" clothes which helps too - he still needs help with the odd awkward button, but rarely now. He's3 years 9 months.
Kindergarten (which starts at 3) expects/ encourages the children to get themselves changed for sport and to put their own coats/ waterproof trousers/ shoes/ boots on to go outside, so the idea he "should" be doing it himself is reinforced there, which can only help.
All my kids have always had firm opinions on their own clothing, from as soon as they could talk, so that probably also helps but isn't something that can be manufactured(eexcept by allowing them choice and buying things they like rather than just what you like).