1a. open whatever poker client you play on (pokerstars, boss, whatever)
1b. open pt3
2. in pt3, select whatever site you are playing on in the import section (pokerstars, boss, whatever)
3. click auto import
4. play poker (you will see a big list of all the hands pt3 is auto importing while you play)
(i only know pokerstars, but other sites may be similar
if its not working, i think you may have to go to the pokerstars lobby in the pokerstars client (where you already are when you open pokerstars) and click on options (i think) and then click on hand histories (or something) and then something like "save hand histories to disk" or "auto save hand histories" or some **** about saving hand histories so they automatically are sent to your computer (you could email stars support if you cant figure it out and they will know what youre talking about). then i think pt3 will know where they are sent and auto import them as you play. if pt3 doesnt know where they are, you would have to go to the configure tab or whatever its called and click on the three dots button and show it where pokerstars is sending the hand histories.
also, on pokertracker.com there is some useful intro stuff under the pt3 heading and under "documentation," but it still doesnt spell out exactly how to do everything. im sure tons of people download pt3 and then just say **** it and give up after not being able to make it work.
the end.