The three dials on the sort of lock you have sound like the type that have a "gate" at the number corresponding to combination. Unless the locks are very well machined slight irregularities will mean that if you put pressure on the release slide button, as if to open the case, one of the dials will get trapped by the lug on the back side of the slide button. Keep the other two dials where they are and advance the trapped dial one number at a time untill it is no longer trapped when you put pressure on the slide. All being well another of the dials will now be trapped and you can repeat the process hopefully finishing with the dials at the right combination.
Good luck