Honestly modern modern LAMs will fit in a 3" handguard since they are around 3" long.
Dbal a3/i2, holosun units, ngal, a peq15 would have to sit back over the receiver about an inch to be flush with the end if the handguard.
But especially if you go to pistol units such as the surefire m400v or xvl2, tlr vir2 (no vis laser), etc those are definitely small enough to fit.