Whenever Thraximundar attacks, defending player sacrifices a creature.
Whenever a player sacrifices a creature, you may put a +1/+1 counter on Thraximundar.
5/1/2009 Thraximundar’s middle ability resolves during the declare attackers step. The defending player sacrifices a creature before it would get a chance to block.
5/1/2009 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.