One of these?
http://www.jaycar.com.au/Power-Produ...upply/p/MP3097
I would not worry, the supply rating is the maximum output the supply can support, if you only use 3A that is not a problem.
I am using one of these
http://www.jaycar.com.au/Power-Produ...upply/p/MP3079
And actually it is the other way around efficiency wise, if you run the supply at a very low percentage of its rated output then there is comparatively greater losses in the supply itself. Whenever I finally manage to build an obs I will probably need more than 12A but until then it makes no sense to pay the money for it, my current gear would draw 5A max and about 3A continuous with the heaters going.