3. D.

Initially, 150 lamps are all on. When the switches for lamps numbered as multiples of 3 are pressed, 50 lamps, which are multiples of 3, are all turned off, leaving 100 lamps still on.

Then, the switches for lamps numbered as multiples of 5 are pressed, changing the state of 30 lamps.

Notice that, 10 lamps are multiples of both 5 and 3, and these 10 lamps are turned back on, while the remaining 20 lamps are turned off.
Therefore, there are 90 lamps still on in the exhibition hall.
3. In a certain exhibition hall, 150 lights are all on, and each light has its own switch. Now the switches are numbered from $1 \sim 150$, and a student first presses all the switches that are multiples of 3, then presses all the switches that are multiples of 5. Then the number of lights that are still on in the exhibition hall is ( ).
(A) 60
(B) 70
(C) 80
(D) 90