What should I look for in a nursery/pre-school for a three year old?

I would ask them if they have a structure to the day - what is the 'timetable'.
(This will usually depend on the physical layout of the building) but I would ask them when, and how often they get to go outside.
I would make sure I clearly understood what I was paying out (most Nurseries have to arrange their finances to get some income around the hours that are free to parents)
I would ask about food - who cooks / prepares (is it a cook, or do one of the room staff nip out and heat something through)
What do they have for snacks ?
Obviously - if it applies - then information about how they meet medical needs.
(Again will depend if your child is toilet trained, but if not) How do they work toilet training
Some people are more anxious than others about security - systems for collecting etc
If you need to leave a pushchair - is there a space you can do that? How secure is it?
Do the children watch TV?
Are there "optional extras" that they charge for (some nurseries have outside providers coming in to offer 'yoga' or french or music or football etc). If you don't want to pay extra for those, what happens to your child at this time. If you do, what is the extra cost.
SOme people get very worked up when they realise they have to pay when the Nursery is closed on a Bank Holiday - it goes back to understanding what the total you pay over the year, is.