Heat very erratic heat comes and goes hot water last just 30 secs

Sounds like you fried an element. For safety reasons never set any hotter than 120 o F.

E3 means that the heating element is operating when it should not be doing so. It may be caused by an older, weaker element that is taking too long to heat the water or the thermostat may be giving incorrect readings. Ensure you are using hot water to fill the machine (if tied to your faucet water supply, run the water until it is hot before starting the dishwasher). Information from the Belling manual:

If the heating element is still heating after the allowed time, then the control board will advance the programme to next step and at the end of the washing cycle the alarm buzzer will sound for 30 sec and the display will show E3.

When the control board detects that the heating element is operating during a non heating step or during the internal test the heating circuit is found to be open circuit then the alarm buzzer will sound for 30 sec and the control board will cease all functions except for the drain cycle which will run for 90 seconds and then E3 will be displayed on the control panel.

Get it scanned for errors. The VSS or the speed sender unit may be damaged or not seated/fitted correctly.

The fan switch could be faulty or worse case is you have combustion fumes being pushed into the cooling jacket. Yup blown headgasket. Take to a reputable shop and have them perform a block test.
The fan could also have an intermittent fault and not kick on all the time when the relay demands it on. Good luck!

Is ther eair in the pump.

replace your stems on the mix handle or hot and cold side.may calcium build up

I have the same problems, did you manage to get it fixed and what was wrong?

check the circulating pump is distributing the hot water, otherwise the heater will switch off because of overtemperature