If a creature is blocked by multiple creatures, can you assign all trample damage to one blocker?

Trample does not let you freely redirect damage to the defending player whenever multiple blockers are present. Under CR 702.19b, a creature with trample must assign damage to each blocking creature in an amount considered lethal before any remainder can be assigned to the defending player (or planeswalker).

"Lethal damage" means damage equal to the blocker's toughness minus any damage already marked on it (CR 702.19b, CR 510.1b). You must satisfy this requirement for every creature that is blocking before a single point of trample damage reaches the player.

You do have flexibility in how you distribute damage among the blockers, provided each one receives at least lethal damage. If multiple blockers have been ordered (CR 509.2), you are not required to assign more than lethal to the first blocker in line before moving to the next — but you cannot skip giving lethal to any blocker in order to push more damage through to the player.

Example: Your 10/10 trampler is blocked by a 2/2 and a 3/3. You must assign at least 2 damage to the 2/2 and at least 3 damage to the 3/3 (lethal for each). Only the remaining 5 damage may be assigned to the defending player. You cannot assign all 10 to one blocker and send 0 to the player via trample, nor can you assign 10 to the player while leaving the blockers unharmed.

In short, trample lets excess damage trample through, but every blocker must still receive a lethal assignment first (CR 702.19b).