According to the American Heritage Dictionary of the English Language, "sibilant" means:

"Of, characterized by, or producing a hissing sound like that of (s) or (sh): the sibilant consonants; a sibilant bird call."

For more information on the meaning, see Answers.com via the Related Link.

some examples include:

SCRIPTS

SCRIMPS

STRETCH

SHRIMPS

STRINGS

All conform to the 3-1-3 format you described.

Although I only used "S" examples,

consonant combinations in 3s range across:

• sch
• shr
• sph
• scr
• spl
• spr
• thr

Hope that helps.