Gaining life means any time that your life total goes up. If your life goes up by more than one life at a time, it is still only one instance you you gaining life. For example, Drogskol Reaver itself will deal three points of combat damage. Because it has lifelink, this causes you to gain three life. However, the Reaver’s ability will only trigger once off of this and you will only draw a single card. Lucky for you, Drogskol Reaver has double strike as well, so assuming it survives the first strike combat damage step, it will deal combat damage again in the normal combat damage step (unless it was blocked by a creature that died during the first combat damage step).
This also applies to other effects that gain you more than one life at a time. If you have less life than an opponent and cast Timely Reinforcements, you will gain six life, the Reaver’s ability will trigger once, and you will end up drawing one card.
Keep in mind that some abilities can cause you to gain more than one life in a short period of time, but each instance is a separate ability resolving that causes you to gain life. For example, Falkenrath Noble has an ability that triggers when a creature dies, which causes the controller of the Noble to gain one life (a target player loses one life as well, but that is not relevant for this example). If more than one creature were to die at the same time, the noble’s ability will trigger once for each of these creatures. Each of these abilities resolves separately, and you will gain life one at a time. Each one of these instances where you gain life will cause Drogskol Reaver’s ability to trigger. In short, you will end up drawing one card for each of these separate instances of you gaining life.
One last thing to be aware of: if you control more than one creature with lifelink, and they deal damage at the same time during combat, the Reaver will trigger once for each lifelink creature that dealt damage. Even though your life total changes all at once, the ability sees each source that caused you to gain life, and triggers accordingly. For example, if you cast the Reaver, then attack with two Vault Skirges, when combat damage is dealt, you’ll gain two life, but the Reaver will trigger twice because the life gain was from two sources.