Does untapping lands trigger Wake Thrasher's ability?
Wake Thrasher is a Rare 1/1 merfolk with a converted mana cost of 3. The value of the card is he gains power and toughness when you untap permanents you control.

From the oracle rulings:


  If permanents you control become untapped during your untap step, Wake Thresher’s ability will trigger that many times. However, since no player gets priority during the untap step, those abilities wait to be put on the stack until your upkeep starts. At that time, all your “beginning of upkeep” triggers will also trigger. You can put them and Wake Thresher’s abilities on the stack in any order.  


Lands are permanents. They untap during your untap step. This would cause the ability to trigger, and "wait to be put on the stack until your upkeep starts." For the three lands you untap, Wake Thrasher will get a total of +3/+3.

This is right.

Edited to add: A second reading of the question highlights the possible consternation. If you are under the impression that the triggered ability of Wake Thrasher puts +1/+1 counters on the creature, which have a measure of permanence, dissuade yourself of that idea post haste. Counters are not put on the creature. Instead, it is temporarily granted +1/+1 untill end of turn. There is a distinction between the concept of +1/+1 counters (MCR 121), and the continuous effect of +1/+1 (MCR 612).