if you've only 3 extension sockets, then it shouldn't be too much of a problem to find the bad connection. try and establsh how they are wired, i.e, start at the first extension after the master and check the connection, if it doesn't work at the first extension, the problem likely lies at the master socket, probably a loose or broken connection, quite common. As they are wired in a chain, any socket after the fault will cause a problem.