A sync chain wears just like any other chain, through mechanical wear of the bushingless pins that hold the links together... so chain elongation (aka, stretch) is checked the same way that it is for a drive chain. On your triplet, the rear sync chain connecting your two stoker's cranks will wear more quickly than the chain that connects your cranks to the middle stoker's, noting that each successive sync chain on a multiseat tandem will carry more load than the one ahead of it.

New sync chains will always show some early wear and sag a bit. In most cases you simply take it up with the eccentrics, adjusting the middle eccentric first and then the front one on a triplet: not sure what the Micargi uses to accomplish this.

Unlike a drive chain, you can basically run a sync chain on the same chain rings until the chain rings look like buzz-saw blades without much of a detremental effect. You can even double the life of your chain rings by flipping them around or rotating them 90* on the crank spiders at their half life when you throw on a new chain. Or, you can make a point of checking your chains for wear using a chain checker tool or a 12" ruler (do a Google search on "sheldon brown chain stretch") to preserve your timing rings. It all depends on how much wear and tear you put into your drive train as to how long they will last and, frankly, inexpensive chains last just as long as expensive ones on a direct drive like a tandem's sync chain.