3. An electronic clock displays time from 00.00.00 to 23.59.59. How much time during a day does the clock display exactly four digit 3s?
3. If the display shows ab.cd.mn, then $a \neq 3$, so there are 5 cases where one of the digits $b, c, d, m, n$ is not 3, while the others are 3. a) On the display ab.33.33, where $b \neq 3$. There are 21 such sets.

b) On the display a3.c3.33, where c is not 3. Here a can take any of the three values 0, 1, or 2, and c can take any of the five values 0, 1, 2, 4, or 5. There are $3 \cdot 5 = 15$ such sets.
c) On the display a3.3d.33, where $d \neq 3$. Here $a=0,1,2, d=0,1,2,4,5,6,7,8,9$, for a total of $3 \cdot 9 = 27$ sets.

d) a3.33.m3 - 15 sets.

e) a3.33.3n - 27 sets.

In total, $21 + 15 + 27 + 15 + 27 = 105$ sets, each of which is displayed for 1 second.

Answer: 105 seconds.