I've been reluctant to say the things I've noticed, as the only ideas I've got about autism/aspergers are those I've picked up from mass media - never the most reliable of sources - and also I've only known a couple of 3-year-old boys so I've not had much to compare his behaviour to. I could have easily got this all wrong.
Anyway, this is what I've seen:
His speech isn't very clear. I find it hard to understand him, compared to friends' children at the same age. A lot of the time he seems to talk through a sort of forced smile, so it all comes out quite 'hissy'
He seems obsessed with numbers. He learnt letters and numbers very early, and has always enjoyed counting, and would frequently just start counting 1-10 repeatedly. On one recent occasion having sat quietly for a long time, not doing much of anything, he stood up and started counting backwards, first from ten - 10,9,8,7 etc., then from 11 - 11,10,9,8, then from 12, until he got to 20, then repeated 20 to 1, louder and louder, until SIL told him to stop.
He is not physically confident, and fairly uncoordinated, though he has recently got very good at running .
I haven't seen him with other children his own age, though this is what SIL is worried about. Apparently he refuses to play with them, preferring to play by himself, and objecting if they approach him and try to play with him. He was almost oblivious to my dd until she started crawling, now he is quite nervous of her and backs away if she ever crawls towards him or his toys.
I don't know, I could be seeing things that aren't there, and misinterpreting toddler behaviour. And I feel somewhat disloyal to SIL typing this all out.