Can Out-of-Order-Sequencing apply to triggered abilities, or will they simply be considered missed? For example, AP controls a Soul Warden. He plays 3 creatures (no responses from the opponent), then announces that he gains 3 life. Does he get his triggers?

I'm inclined to say yes. The player clearly knows that he was supposed to gain the life, he was simply taking legal actions in a different order for convenience, which is exactly what OoOs is designed to cover. The IPG also includes the following sentence in section 2.1 (missed triggers):

The Out-of-Order Sequencing rules (MTR section 4.3) may also be applicable, especially as they relate to batches of actions or resolving items on the stack in an improper order.

However, some judges have disagreed with me over this. It's also interesting to note that while 2 of the examples of OoOs in the MTR include triggered abilities, they both pertain to resolving the trigger before it was supposed to resolve, not after.

I found a couple of old articles which touch on the topic, but neither of them have a definitive answer. They are also rather old and refer to out-of-date missed trigger policy, so I'm reluctant to trust them:

http://blogs.magicjudges.org/rulestips/2013/03/understanding-out-of-order-sequencing/
http://blogs.magicjudges.org/telliott/2012/12/14/out-of-order-sequencing-and-triggers/

Edited Isaac King (July 8, 2016 05:36:05 PM)

I would probably check with the players what the pace was like. If it was tapping correct mana, playing the three creatures simultanously and immediatly say “gain three life”, I would say OoOS is correct.

If the pace was clearly slower and the creatures was played one by one with a clear pause between them, then all except the last (which could still be on the stack) would be missed triggers.

Out of order sequencing can apply in determining if a trigger is missed. It's called out explicitly in the philosophy section of Missed Trigger.