Generic case problem

Measure diagonally from the top left screwhole to the bottom right screwhole. That will show you how large the fan is. Then you can search by fan size. Most common sizes are 80mm, 92mm, 120mm & 140mm.

Without seeing dimensions no one is going to be able to guess the size of the fan.

the inside circle is 14cm diagonally, but there are 3 screwholes, what type of fan can be fit there

It would be helpful to see a wider shot of the fan mount. I can't really make heads or tails of what I'm seeing. But if your measurement is accurate you'd be looking at 140mm fan.