I have a Volvo that is giving me problems with cranking. At first I thought it was the battery losing charge overnight but recently it left me stranded at work after driving a couple miles. I was able to jump it to get home. After arriving home it would not crank. I turn on the key and the dash lights come on and the radio works. When I turn it to start the lights go off and the radio goes off. I disconnected the starter so there is no load on the battery when starting and it does the same thing. I also checked the starter voltage and there is nothing there when turning to start. I connected a jumper directly to the starter and it cranks just fine. What is causing the lights to go off when trying to start? Is there a load somewhere that's loading down the battery? I have three wires on my battery. What are they for? Any help would be appreciated.

Thanks, Larry