Why do you need three meters? The other day I went out to shoot a concert. While hitching the main spotmeter to my belt the loop came out and meter crashed to the ground. Thought it worth checking and odd. No spot. Incident was fine but spot was reading UEX. No worries, I'll get out the trusty Spotmeter V. Pulled it out and aimed. No spot. Dead'ern a doorknob. No worries, I've got spare batteries in the ESII... Popped 'em in and nothing.
Well, I'm not about to take the SEI to a concert. So I just went without meter at all.
Came home after and found I'd put the batteries in upside-down in the Spotmeter V... it was fine. The main meter turned out to be easy to fix as well. A circuit board had popped off its socket...
So although you only need one meter. Having three could come in handy some day.