The following method for finding the three letter lexical verb form was recommended to me by my Hebrew professor at Baylor University, Dr. Edward Dalglish. Follow the flow chart, and the three letters can be identified for more than 95% of Hebrew verbs that appear in the Bible. The exceptions (several of which are identified on the chart) are well known verbs which are usually recognizable without the aid of the chart. If the chart looks a bit complex, remember that it takes the place of multiple chapters in a textbook. Also remember that this chart is for identifying verbs so they can be read - I wouldn't recommend this approach for people who are attempting to become conversational in Hebrew.
Click on the chart to see the whole thing.