For the plugs all three countries use the standard 2-pin round European plugs (they were French colonies after all!), and whilst some of the hotels we use may have UK 3-pin adaptors built in a European adaptor would be useful.
With regards to currency sterling is widely accepted and if you don't have any dollars already it's not worth changing sterling to dollars in the UK only to change when you're over there. Some dollars will, however, be necessary in Cambodia for purchasing the visa for example, so if you have some lying around then it's a good idea to take them too.
I hope that helps,