controlling heating element using microcontroller
Why are you using two of the SSR modules (solid state relays)? 3000 Watts supplied from 240V RMS is about a load of about 12.5 Amps RMS. This should be an easy load for the CMRD2435 to handle. 

If you were paralleling two SSRs to share a larger load than either one was rated for then it is feasible that they would not turn on and off at the same time and thus one unit could end up being overloaded whilst it was momentarily bearing the brunt of the load. 

It could be that the pressure relay is pulsing on and off rapidly when the SSR is on. If the load has an inductive component to it this can generate huge high voltage spikes that are a death nail for an SSR. Your SSR is rated for a peak of 600V peak transient. An inductive load could be generating spikes well over that level. 

You should check your setup for the presence of spikes using an oscilloscope. If large spikes are present you will have to look at adding MOV type clamp components and possibly some R/C snubber circuits across the the heater load to suppress the large spikes.