How do you know if you need a new clutch if the gears change all right and the clutch doesnt slip but you have loss of speed when you change into third gear or go uphill?

If the revs decrease as the speed decreases (in 3rd & uphill) then its not slipping, you don't need a new clutch, but if you are in any gear and the revs are increasing while the speed is decreasing, then the clutch is slipping.