leaves are yellow and falling off
They are watered 3-4 times a week, fertilized every 3 weeks and have full sun all day.
Thank you for letting me know it's a hibiscus.
Yellowing leaves are very common and usually not a lot to worry about.
Here is a link that has good information about common causes of yellowing leaves.
Did you recently move the plant? It does sound like your care is appropriate, but always check the soil before watering. Sometimes we overwater out of habit, or maybe it's needing more water as the spring progresses with warmer temperatures. Make adjustments if need be.
Could you let us know the type of plant you are referring to?