Instructions for 3T can be found by clicking here.
It consists of picking three tasks and working on them in rotation until two of the tasks are completed. Then another two tasks are picked and once again one works on the three tasks until two are completed.
It is designed to ensure that tasks are worked to completion, and can be used on its own or using another system to feed it.
Progress can be very slow, especially if more than one of the three tasks will take some time to finish.
3-T is not really a complete system, but can be very effective in clearing projects which have got stuck. It drew attention to the desirability of working tasks to completion, and later systems have found better ways of doing this.