A 24 Volt battery pack can only be made with an even number of 12 Volt batteries, such as with 2 or 4 batteries, and it can not be made with an odd number of batteries. If you have three batteries you will need to remove one of them and only use 2 batteries, or add an extra battery and make a 24 Volt battery pack out of 4 batteries.
Here is a link to our battery pack wiring directions page: http://www.electricscooterparts.com/battery-pack-wiring.html
Please let us know if you have any questions.
Jamie