Hi Richard and welcome to GoY. Where are you in the UK, what is your soil like, acid, alkaline, neutral and what aspect does the border have? Sorry for all the questions but we will be able to give you better advice if we know. If you can add a photo to your question so we can see the border that would be great.

The cheapest way to buy hedging is bare root, supplied late autumn. Plants sold at this time of year will be potted, and that makes them more expensive.

If you want a hedge at 3 feet, do you mean a hedge you keep trimmed (formal) or something that can be left to its own devices (informal, or messy as some people describe it)?

As MG says above, information about what part of the country you're in, and how exposed this border actually is, if there's a fence behind it or something, whether it faces north or east or whatever, are important bits of info to know when selecting plants.

Agree with above, especially buying 'bare-rooted'. Copper Beech will make a good hedge within about 3 years, or you could plant a mixture (as we have done) of Cotoneaster, Choisya, Berberis, Euonymus and Photinia 'red robin' which have merged together.