Haste
Whenever Thraximundar attacks, defending player sacrifices a creature.
Whenever a player sacrifices a creature, you may put a +1/+1 counter on Thraximundar.
Thraximundar’s last ability triggers whenever any player (including you) sacrifices a creature for any reason. It doesn’t trigger only when a creature is sacrificed due to its middle ability. Note that Thraximundar itself doesn’t allow you to sacrifice any creatures.
Thraximundar’s middle ability resolves during the declare attackers step. The defending player sacrifices a creature before it would get a chance to block.