What is the effect if a target (A) is grabbing another target(B), and another person(C) applies 3 squares of
Is the grab-victim (B) carried along with (A) for the trip?
The general rule is that A is moved away without B, and if the movement takes B out of A's melee reach, the grab ends. References can be found in the definition of the grabDDI action at page 290 of the Player's Handbook and after the definition of the grabbed condition on the Essentials Rules Compendium (page 231).
As a minor side note, grapple is a term from previous editions. 4th edition has the much simpler grab condition (I supposed you intended that one in the question).
Forced movement on one combatant does not move another due to a grab by default.
The definition of Grab contains:
If a pull, a push, or a slide moves you or the creature you’re grabbing out of your reach, the grab ends.
Contrast this with the RestrainedDDI condition, which does prevent forced movement.
Pushing an grabber away from a target he has grabbed usually breaks the grab. There are a few exceptions, where the grab is not broken and the grabbed target gets brought along for the ride, in some cases of special enemies like the Gelatinous Cube [ddi].
Engulf (acid) At-Will
Requirement: The cube must have no more than two creatures grabbed
Attack: Melee 1 (one or two creatures); +8 vs. Reflex
Hit: The cube grabs the target (escape DC 15) and pulls the target into its space. Until the grab ends, the target takes ongoing 10 acid damage and is dazed. When the cube moves, it pulls with it any creature grabbed by it, and the creature remains grabbed and within the cube’s space. This movement does not provoke an opportunity attack from the grabbed creature.