Hi, how's your RES doing?
Constant yawning is one of the many symptoms of possible respiratory infection.
Have you noticed any other symptoms such as constant basking, swimming lopsided or bubbling at her nostrils?
Could you show us some pictures of her living condition?
If you're prepared to bring her to see a vet, pm me and I'll share a herp vet located in Ampang.
Good luck!

From my experience, local fish shops that sell these turtles are generally not too concerned about proper living conditions for them, so we shouldn't use their set ups as a benchmark.

Having said that, I too believe your set up is good enough, at least until these fellas outgrow your tank, which would be sooner than you think.

If I'm not mistaken, RI is contagious to the other turtles, so you may need to start segregating the guy with it from the other 2.

Perhaps it's also prudent to purchase a thermometer to monitor their water temperature, a cheap one should do.