The thing to understand is that the number of Watts determines how much heat is put into the water. More Watts means faster heating, The thermostat will prevent the water getting too hot by cutting the power when the pre-set temperature is reached so the Wattage decides how quickly it warms up rather than how hot it gets.
The heaters you show are rated at 500W where the original ones were 300W so to get the same heating effect you only need two instead of three. Be careful because if you follow the original design it describes three holes to mount them and obviously, you need to rearrange them slightly so you have two side by side instead of three in a triangle.
The two will give you 1000W of heating (2 x 500W) where the original used 900W (3 x 300W) but the difference is negligible. Other than those cautions they should work fine.
Brian.