Read as much as you can about the autistic spectrum, communication methods such as PECS, Makaton and about Social Stories. You need to realise that it is a spectrum, someone can be on that spectrum at some point in it, over time they may move within the spectrum - such as a non-verbal child develops verbal language.
The bond between them and you I feel is important, they need to trust you. You need to be able recognise when things are getting too much for them and to pull the out of a situation before it kicks off. They need you to know of any triggers and thus to avoid those occuring.
Age 3 in my view is quite early for a diagnosis. The professionals involved may not know at this stage where the child is on the spectrum, or they may have a good idea and may have some intervention things they want tried, such as ABA.
For now, I would not worry about if they have a diagnosis or not... they have enough to get a 1:1 so you need to focus on why you are there, what is it that is aiming to be achieved. Little steps, achievable goals. Look at his current interest... Trains for example. Use that interest as a stepping stone, so in the case of trains you can use it to learn colours, some shapes, sharing with others, sounds, all sorts of things - topics can be expanded greatly sometimes.
Routines and consistancy may be very important to him. If he has to do a certain thing, then write it as a routine using pictures. www.pictoselector.eu/ can be used for creating pictorial routines.
This could be good for circle time... can you use pictures to describe what happens at circle time? Is there always something or multiple things that precede circle time... so could you create a routine for that bit of the day so he knows what is coming next. Does he really need to sit through circle time, is circle time too long? Things you will need to discuss with the pre-school leader... they may be able to adapt things, make it a fixed length for example.
Be aware of how close he will let people, including you be to him. Different people may be allowed a different distance. He may not like touch, or may let some people touch him but only in certain places.
Children on the spectrum can be sensitive to touch, sound, temperature. Be aware of what he likes, dislikes, reacts to.
One reason you are there could be to protect other children, so you do need to be quite close but as you say you don't want to be in his face.
Talk to the supervisor. Talk to his parents, what do they want achieved? Your local Early Years department may offer some SEN courses, so ask your supervisor about training opportunities.