:3 is an emoticon used to express sillyness or playfulness. Often by furries or those who spend time talking to furries via text, or those who spend time talking via text to those who spend time talking to furries via text, etc. You get the idea, it originated with furries, but unlike the (in)famous OwO and UwU, it spread much further and is used by more than just furries and adjacent groups. It is supposed to look like the face of a cat. (rotated sideways ofc)
Variants: (by order of how often I've seen them used)
>:3 Used to express mischeif, or (usually joking) intent to commit mischeif.
X3 Laughing variant. (equavelent to X) or XD)
:3c Variant with paw1
😺/😸 {Warning: Small sample size} In my experience, it's rarely used, and even then, mainly used to indicate actual cats, and only rarely used similar to the normal :3.
;3 Winking variant.
=3 I've never seen someone use this in the same way as :3. Sometimes used in comics to indicate a sigh, although then it's usually at an angle.
1IT IS NOT a compact variant for use by special forces & vehicle crews like the G36C, capital C vs lowercase c.