I do not know your model, but will offer this as your description sounds similar to my arrangement.
You removed 3 bolts and lifted off the upper housing cover. You're likely having a tough time pulling out the thermostat because of the thermostat gasket that runs around the circumference of the inner part of the lower housing. This gasket must first be removed. I use a sharply pointed object to gently dig it out. Once removed, the thermostat pulls out easily.