Nasm, 3 characters.

any of the following:

ret
nop
cbw
cwd
cdq
clc
cld
cli
cmc
aaa
aas
aad
aam
stc
std
sti

ret raises SIGSEGV via stack underflow. All the other opcodes raise SIGSEGV simply because there's no exit point.

NASM, 3 characters.

any of the following:

ret
nop
cbw
cwd
cdq
clc
cld
cli
cmc
aaa
aas
aad
aam
stc
std
sti

ret causes stack underflow. cli and sti trigger SIGSEGV because they are privileged instructions. All the other opcodes trigger a SIGSEGV simply because there's no exit point.

Nasm, 3 characters.

any of the following:

ret
nop
cbw
cwd
cdq
clc
cld
cli
cmc
aaa
aas
aad
aam
stc
std
sti

ret raises SIGSEGV via stack underflow. All the other opcodes raise SIGSEGV simply because there's no exit point.

Nasm, 3 characters.

any of the following:

ret
nop
cbw
cwd
cdq
clc
cld
cli
cmc
aaa
aas
aad
aam
stc
std
sti

ret raises SIGSEGV via stack underflow. All the other opcodes raise SIGSEGV simply because there's no exit point.

Nasm, 3 bytes.

Either

ret

or

nop

ret raises SIGSEGV via stack underflow, and nop raises SIGSEGV simply because there's no exit point.

Nasm, 3 characters.

any of the following:

ret
nop
cbw
cwd
cdq
clc
cld
cli
cmc
aaa
aas
aad
aam
stc
std
sti

ret raises SIGSEGV via stack underflow. All the other opcodes raise SIGSEGV simply because there's no exit point.