I've got a couple techniques that I use to 'sweep' 3 to 4 strings. Try (on a 4-string) putting your ring finger on the E string, middle finger on the A string, and pointer finger on the D string. When in this position, hold the notes, for example,
E7 A9 D9, then successively hit each string starting with the E string. This is a sweep up - you can also do the same thing down by putting your ring finger on the D string, middle finger on the A string, and pointer finger on the E string.
You can incorporate a fourth string and your pinky finger also... its just a little more difficult because your pinky finger isn't that long and you need to adjust your wrist angle to pull it off.
Another technique I like to use is to sweep up like described above and then with the pointer finger, sweeping back down all the strings.
E7 A9 D9 A9 E7. You can also do the reverse by sweeping down like described above and then back up with your finger nails.
D9 A9 E7 A9 D9.
It's hard to explain in words, but you can hear me using these techniques in some of my songs. There is a link in my profile... if you are interested, listen to Karmageddon and The War. The techniques are also used in the other songs, but not as heavily. Don't really mean to "plug" my music, but it is relevant in this case I suppose.