I don't do a nanny share and never have done, so can't advise on how it works in practice. However, I can give some general advice.
A nanny wouldn't have any issues with taking or collecting children from school/nursery. When you employ a nanny, you are usually employing them for the whole time, including the time your child is at the nursery.
If you were to use a Childminder, then they would be more likely to provide care only before and after school - however, that's when the children are school aged (10 educational sessions). With younger children, they will take up a under5 childminding place and thus you are likely to be charged for the entire time, as it's very hard to resell the time while your child attends nursery/pre-school.
You need to remember that while your child attends a local nursery/pre-school, there needs to be someone oncall in the event of any issues - if you will be available to collect your child at short notice, then your childcarer will need to be available.
Pay wise, I would expect that for 2 children, £8 per hour would be reasonable. If 3 children, increase to £12. Certainly in my area Childminders tend to charge around £4 per hour. With 3 children, a nanny can work out to be cheaper, as you are more in a position to offer a salary, thus could offer say £10-£11 gross.
As you would be working only 3 days, you could get a 3-day nanny. Sharing that nanny with another family may also be possible.
With children who live near each other, going to the same pre-school/nursery, once the children become friends you do share the school runs a bit. As a nanny, on occasion I used to take another child to pre-school to help out the other parent, and they in turn would on occasion take/collect the child I cared for. When doing this sort of thing, you need to make sure that the nursery/pre-school know who is collecting the children.
Important: You can't do a Nanny Share if there are 3 familes involved. \link{http://www.opsi.gov.uk/acts/acts2000/00014--g.htm\Care Standards Act} - see 4b for definition of nanny share (sorry, it's a bit complex!) The Care Standards Act does not say that in the event of there being an arrangement between P1, P2 and P3, that such an arrangement would be exempt from Childminding registration. Thus, I conclude that if you nanny share between 3 families, then the nanny MUST be a Registered Childminder, though in reality I feel that Ofsted may have trouble in processing such an application. So avoid having a 3 family spit.
Hope that's of some help.
Where abouts in the country are you? Have you establish what sessions your child would get at nursery - as sessions are not always in the morning.