As the Gatherer rulings on Quietus Spikes say:
(10/1/2008)
If multiple Quietus Spikes trigger at the same time, that player loses half their life when the first ability resolves, then loses half of the remainder when the next ability resolves, and so on. The player does not lose the same amount each time.
It’s identical in your situation; both are triggered abilities, they will be separate events on the stack, so they’ll resolve independently. In this case, two halves make three quarters, not a whole.
Note that because of the ’rounded up’ in the card text, if their life total is 3 or less after combat damage is dealt, that’ll be enough to put their life total to zero anyway (3 → 1 → 0, 2 → 1 → 0, 1 → 0).
Incidentally, in very special cases, the last sentence of the Oracle text is incorrect, namely when the affected player has life total 0 (or lower), but is still in the game due to something like a Platinum Angel. In that case, they’ll lose an equal amount of life, namely 0, both times.