First off none of these abilities trigger, these are all replacement effects.
Secondly, yes, replacement effects can modify other replacement effects. Both Paragons make your warriors come into play with an extra +1/+1 counter, each of those effects is modified by Scales to make two counters be placed on that warrior.
Glistener Elf will come into play with 4 +1/+1 counters on it.
August 31, 2016 9:38 a.m.
sonnet666's answer isn't entirely accurate. As they said these are replacement effects, and according to 614.5 a replacement effect can only replace an event once. The event that is being replaced is the Glistener Elf entering with zero counters.
Initially only the two Bramblewood Paragon replacement effects can apply to this, you chose one of them and it applies. Now the Glistener Elf is entering the battlefield with one counter and the second Bramblewood Paragon replacement effect can still apply and the Hardened Scales replacement effect can apply because one or more counters is being placed.
From here you have two options but they both have the same outcome. You can apply the second Bramblewood Paragon replacement bringing Glistener Elf up to entering with two counters and then Hardened Scales bringing it up to three. You could also apply the Hardened Scales first and then the second Bramblewood Paragon this brings the total number of counters up to three as well. You apply each replacement effect once and only once to the event of the Glistener Elf entering the battlefield regardless of the order.
614.5. A replacement effect doesnt invoke itself repeatedly; it gets only one opportunity to affect an event or any modified events that may replace it.