No-I, the third Sign, is a little more difficult.
Buddhists do not believe that there is anything everlasting or unchangeable in human beings, no soul or self in which a stable sense of 'I' might anchor itself. The whole idea of 'I' is in fact a basically false one that tries to set itself up in an unstable and temporary collection of elements. Take the traditional analogy of a cart. A cart may be broken down into its basic components -axle, wheels, shafts, sides, etc. Then the cart is no more; all we have is a pile of components. In the same way 'I' am made up of various elements or aggregates (khandhas): form (rupa-khandha), feeling-sensation (pleasant, unpleasant, neutral), (vedana-khandha), perception (sanna-khandha), volitional mental activities (sankhara-khandha), sense consciousness (vinnana-khandha).