This is the thing Cyberseraphim, I was talking to my mum about him and she says that she was in a similar situation with me at that age, in that there were things which made her think and things that in hindsight she realised were significant, but at the time there were enough things counterbalancing. Eg:
Both Ds2 (and myself at that age) are/were very clingy with respective mums. But so are lots of very young children.
Ds2 says very few words and has very poor pronunciation, but good understanding. I spoke rarely and spoke very fast when I did speak, but also had good pronunciation.
Ds2 likes to line things up, but so do a lot of three year olds.
He has quite a poor grip and takes little interest in things which require manual dexterity, eg tower building or scribbling/painting. But this isn't necessarily something to be concerned with at this age and he is starting to enjoy glueing things at least.
He is very selfcontained but does readily and happily approach myself and interact with me and with DH and to a lesser extent Ds1.
So it really all boils down to at what point do we say ok, let's get him seen and checked out. Because at this stage there isn't anything clear cut, yes he has speech delays and yes he is very quiet and yes he does prefer being on his own than with other children, but there are lots of other three year olds like that and I don't want to fall into the trap of thinking it's more than that because of myself and Ds1. But at the same time I can't see any harm in seeing if he needs speech therapy at least and if in a few years time other things become more apparent, or more of an issue then at least we'll have noted his early years.