I am working on an advert rotator somewhat similar to facebook. . .i have done the forms. . . .submitted to dbase and all.

Now, i am operating on pay-per-click and pay-per-view. . . .so now, i am not quite sure of how to rotate the adverts. .
by the way, i am displaying 3 adverts at a time just like facebook. . .

I can use wondering if i should use cron job and all stuffs. . .i have not quite done this before. . .i am seeking for advice
on how best to go about the display. . .explanation / algorithm is what i need, not codes . . . . .