lol,
This is something that little people actually understand,
say for example blinds are 100/200 a 3 bet on the BB would be 600 total,
now for example blinds are 100/200, someone opens for 500 total (a raise of 300 and a 2.5 bet) a 3 bet in this situation would be 3 x the raise portion of the previous raise so 3x300+200=1,100 total. A lot of people think a 3 bet or a 4 bet are just 3x/4x the total bet being faced, so in the example above a lot of people think a 3 bet on the 500 total is 1,500.
A 3 bet is made on the previous bet or raise (not total where multiple action has occurred) (the same way a raise must the size of the previous bet or raise (eg 100/200 raise to 500, next raise can be a minimum of 300 more to 800 total)
Hope this helps.
Stuart