Hi. The tank is actually too small for 1 terrapin let alone 3. The usual guideline for tank size is 10 us gal per inch of shell of each individual terrapin. The water is also too shallow as terrapins are natural swimmers and should have a water depth that allows them to swim freely and submerge fully.
Submerge fully when they are vertical.
They also should not have gravel, terrapins tend to be curious and might swallow the gravel and cause an impaction - the gravel will get stuck in their digestive tract and will likely choke them.