First flip flop i.e. lsb changes every clock cycle. Since 3 flip flops are in cascade, it requires minimum 3*Tpd for all the 3 bits i.e state to be stable.... If at any time<3 times Tpd, again clock is applied..... Lsb again toggle and the counter will still work but will not act as an up counter or a down counter, since there is no stable state... We only get undesired values as states...... Therefore for proper working as an up counter or a down counter, Tclk should be greater than Or equal to 3*Tpd.....