Initially $\rightarrow 000$
- After $1$st clock $\rightarrow 111$ . How ?
The first FF will always toggle on every input .Now, we goto $3$rd FF and take feedback from $Q_{2}' = 1$. This $1$ reaches $2$nd FF and $Q_{0}$= clock input = 1 makes the $2$nd FF toggle giving $Q_{1} = 1$ = input for $3$rd FF with clock being high makes it toggle too.
This gives the next state $111$.
- After $2$nd clock $\rightarrow 011$
- After $3$rd clock $\rightarrow 110$
- After $4$th clock $\rightarrow 010$ . How ?
The first FF will always toggle on every input .Now, we goto $3$rd FF and take feedback from $Q_{2}' = 1$. This $1$ reaches $2$nd FF and $Q_{0}$ = clock input = 0 makes the $2$nd FF not toggle giving $Q_{1} = 1$ = input for $3$rd FF with clock being $0$ makes it not toggle too.
- After $5$th clock $\rightarrow 100$
- After $6$th clock $\rightarrow 000$
