I was best answerer in three categouries, but was out for three months !?

You gain them back the same way you earned them in the first place ... by participating in the categories in which you hope to gain the TC badges.

You lost the TC badge because your participation decreased (or stopped altogether). To regain one (or 2 or 3), you need to increase and FOCUS your answering on those categories. I've had as many as 3 at once, and lost 1, 2, or all 3 because I just get bored with answering the same questions over and over again.

To get the TC badge back, you have to do the same things as the first time. It is a badge of RECENT activity, so being out three months will cause you to lose it. Just start answering and getting BAs in those categories. It will take 2-4 weeks to get it back.

Badges come and go with each new Monday. Yahoo keeps the formula secret (so they can change it if needed). The best guess based on experience of long time users is that you need:

200 or more total points per week

30-50 answers per week in a single category

20 or more BAs in the category within the last 3 weeks (10-15% most people think)