Difference between multi slats and three slats on B737 ? (leading edge devices)
Prior to the NG, the total number of slats is 6, that is 3 slats per wing

Thus there are 6 slats numbered from 1 to 6, that is slats 1 and 6 are the outer slat

The NG's have an extra outboard slat on each wing giving 8 in total. They have the same sequencing as the classic

Thus a 3 slats per wing refers to 737 prior to the NG

more precisely

Leading edge devices are comprised of 4 Krueger flaps inboard of the engines and 6 slats outboard of the engines. The LE flaps are extended whenever the TE flaps are not up. The slats extend from 1 to 5 and fully extend when beyond 5. Slat numbers 1 &amp; 6 (the outboard slats) move a few degrees less than slats 2 to 5 when at full extend, causing the leading edge to look slightly disjointed in this configuration, this is normal.