Go to Tailor > Divisions > Set Top Finishers:
Select whether you want to define Top Finishers as a default for all Division, or one-by-one if they vary by Division.
The next step can get complicated if you have a more complex scenario, but for your's you will just need one overall band that sets aside the top 3 males and females from their Age Groups and puts them into an Overall band.
Click Add Band then adjust the tic boxes under Male and Female to 3 for each, then click Save.