The Effortless Concentration feat in its descriptions across multiple caster classes appears to have an error that is not corrected in Errata #1.
In the CRB (e.g. page 201) it's marked with the free action glyph, but it has a Trigger listed and its description also suggests it's meant as a reaction, not a free action. I assume the intent is that at the start of your turn, you can elect to spend 1 reaction to sustain 1 spell.
Reactions don't come into it.
What the feat is giving you, in plain English, is "you gain one Sustain for free each round", effectively allowing you to sustain up to four spells instead of just three.
The specific implementation is designating the ability a "free action". Since all actions including free ones are governed by the general rule that any given trigger can only be used for one action; i.e. you can't trigger (the verb) more than one thing from any given trigger (the noun), this means that as long as the requirement is properly worded, it restricts you to one use per turn.
In this case, it's "You haven’t acted yet on your turn." which makes it a pretty slam-dunk case: as soon as you use the feat and make a free sustain, you have started to act on your turn, preventing any more fee sustains that turn.
In short: triggers ≠ reactions.
A reaction is just an action with a trigger, just like any other action. It's just that the specific rule "only one reaction" allows the trigger requirements to be lax. For instance, a requirement of "anytime you blink your eyes" is okay for a reaction, since no matter how many times you blink your eyes, you still get only the one reaction. On the other hand, it wouldn't work for a free action, since few GMs would argue you couldn't blink your eyes and thus get the free actions several times during a round.
Mods: feel free to edit the thread title - no errata is needed here.