( Please avoid putting your entire post in code tags. Just put <p> at the start of every paragraph instead. )

You didn't specify byte order and whether the numbers are signed or unsigned.

@fields = unpack('n3', $_);   # unsigned big-endian
@fields = unpack('v3', $_);   # unsigned little-endian
@fields = unpack('S3', $_);   # unsigned native
@fields = unpack('n!3', $_);  # signed   big-endian
@fields = unpack('v!3', $_);  # signed   little-endian
@fields = unpack('s3', $_);   # signed   native
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You provided even less information about the strings.

• Is the field fixed width or variable width.
• If the field is fixed width, how do you know how much of the field is the string?
• If the field is variable width, how do you know how many bytes the string is?
• What character encoding is used? (Did you really mean ASCII?)

Update: n! and s! were introduced in 5.10. Pre 5.10,

# signed big-endian
@fields = map { unpack 's', pack 'S', $_ } unpack('n3', $_);

# signed little-endian
@fields = map { unpack 's', pack 'S', $_ } unpack('v3', $_);
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Update: Oops, had S for both signed and unsigned at the top. Thanks AnomalousMonk.