Yes. One of the rulings for Drogskol Reaver (click the autocard link and scroll down to the bottom) even points this out explicitly.
"If multiple creatures with lifelink you control deal combat damage at the same time, the damage dealt by each of those creatures is a separate life-gaining event and Drogskol Reaver's ability will trigger that many times."
In the first combat damage step, drogskol reaver deals damage, trigging the ability once. In the second combat damage step, all three creature deal damage, triggering the ability three times due to this rule:
118.9. Some triggered abilities are written, "Whenever [a player] gains life, . . . ." Such abilities are treated as though they are written, "Whenever a source causes [a player] to gain life, . . . ."If a player gains 0 life, no life gain event has occurred, and these abilities won't trigger.
Example: A player controls Ajani's Pridemate, which reads "Whenever you gain life, you may put a +1/+1 counter on Ajani's Pridemate," and two creatures with lifelink. The creatures with lifelink deal combat damage simultaneously. Ajani's Pridemate's ability triggers twice.