@sumeet inani: yes -k3n is a sort of combination of two options - as pixellany pointed out. However the start position and the end position can be more detailed, as explained in the man page when it talk about POS:
start a key at POS1, end it at POS2 (origin 1)
POS is F[.C][OPTS], where F is the field number and C the character position in the field;
both are origin 1. If neither -t nor -b is in effect, characters in a field are counted from
the beginning of the preceding whitespace. OPTS is one or more single-letter ordering
options, which override global ordering options for that key. If no key is given, use the
entire line as the key.
In -k3n we have F = 3, C = undef and OPTS = n.