You could do saltwater in this tank, but there are some caveats. First, three gallons is very small, which means that water quality will change very quickly. Tanks of this size are generally considered for "experts only", since someone very familiar with saltwater systems would be able to anticipate/recognize problems before they get out of hand, while a novice will just wonder why his/her fish died. A second issue is that at this size you could house only one or perhaps 2 very small fish. When we say small, we mean size as adults. Many novices purchase fish that will eventually be too large for their tank, but promise themselves that they will remove the fish to a larger system as it grows. This almost never works out, since the fish are experts at avoiding capture, so removing one generally involves tearing down the rockwork. In the meantime the growing fish is becoming more psychotic by the day as a result of being housed in a system just too small for it.
